opinion: Mr. Justice White delivered the opinion of the Court.
  Burns International Security Services, Inc. (Burns), replaced another employer, the Wackenhut Corp. (Wackenhut), which had previously provided plant protection services for the Lockheed Aircraft Service Co. (Lockheed) located at the Ontario International Airport in California. When Burns began providing security service, it employed 42 guards; 27 of them had been employed by Wackenhut. Burns refused, however, to bargain with the United Plant Guard Workers of America (UPG) which had been certified after a National Labor Relations Board (Board) election as the exclusive bargaining representative of Wackenhut’s employees less than four months earlier. The issues presented in this case are whether Burns refused to bargain with a union representing a majority of employees in an appropriate unit and whether the National Labor Relations Board could order Burns to- observe the terms of a collective-bargaining contract signed by the union and Wackenhut that Burns had not voluntarily assumed. Resolution turns to a great extent on the precise facts involved here.
  I
  The Wackenhut Corp. provided protection services at the Lockheed plant for five years before Burns took over this task. On February 28, 1967, a few months before the changeover of guard employers, a majority of the Wackenhut guards selected the union as their exclusive bargaining representative in a Board election after Waekenhut and the union had agreed that the Lockheed plant was the appropriate bargaining unit. On March 8, the Regional Director certified the union as the exclusive bargaining representative for these employees, and, on April 29, Wackenhut and the union entered into a three-year collective-bargaining contract.
  Meanwhile, since Wackenhut’s one-year service agreement to provide security protection was due to expire on June 30, Lockheed had called for bids from various companies supplying these services, and both Burns and Wackenhut submitted estimates. At a pre-bid conference attended by Burns on May 15, a representative of Lockheed informed the bidders that Wackenhut’s guards were represented by the union, that the union had recently won a Board election and been certified, and that there was in existence a collective-bargaining contract between Wackenhut and the union. App. 4-5, 126. Lockheed then accepted Burns’ bid, and on May 31 Wackenhut was notified that Burns would assume responsibility for protection services on July 1. Burns chose to retain 27 of the Wackenhut guards, and it brought in 15 of its own guards from other Burns locations.
  During June, when Burns hired the 27 Wackenhut guards, it supplied them with membership cards of the American Federation of Guards (AFG), another union with which Burns had collective-bargaining contracts at other locations, and informed them that they had to become AFG members to work for Burns, that they would not receive uniforms otherwise, and that Burns “could not live with” the existing contract between Wackenhut and the union. On June 29, Burns recognized the AFG on the theory that it had obtained a card majority. On July 12, however, the UPG demanded that Burns recognize it as the bargaining representative of Burns’ employees at Lockheed and that Burns honor the collective-bargaining agreement between it and Wackenhut. When Burns refused, the UPG filed unfair labor practice charges, and Burns responded by challenging the appropriateness of the unit and by denying its obligation to bargain.
  The Board, adopting the trial examiner’s findings and conclusions, found the Lockheed plant an appropriate unit and held that Burns had violated §§ 8 (a)(2) and 8 (a)(1) of the National Labor Relations Act, 49 Stat. 452, as amended, 61 Stat. 140, 29 U. S. C. §§ 158 (a)(2), 158 (a)(1), by unlawfully recognizing and assisting the AFG, a rival of the UPG; and that it had violated §§ 8 (a) (5) and 8(a)(1), 29 U. S. C. §§158 (a)(5), 158 (a)(1), by failing to recognize and bargain with the UPG and by refusing to honor the collective-bargaining agreement that had been negotiated between Wackenhut and UPG.
  Burns did not challenge the § 8 (a) (2) unlawful assistance finding in the Court of Appeals but sought review of the unit determination and the order to bargain and observe the pre-existing collective-bargaining contract. The Court of Appeals accepted the Board’s unit determination and enforced the Board’s order insofar as it related to the finding of unlawful assistance of a rival union and the refusal to bargain, but it held that the Board had exceeded its powers in ordering Burns to honor the contract executed by Wackenhut. Both Burns and the Board petitioned for certiorari, Burns challenging the unit determination and the bargaining order and the Board maintaining its position that Burns was bound by the Wackenhut contract, and we granted both petitions, though we declined to review the propriety of the bargaining unit, a question which was presented in No. 71-198. 404 U. S. 822 (1971).
  II
  We address first Burns’ alleged duty to bargain with the union, and in doing so it is well to return to the specific provisions of the Act, which courts and the Board alike are bound to observe. Section 8 (a)(5), as amended by the Labor Management Relations Act, 1947, 29 U. S. C. § 158 (a) (5), makes it an unfair labor practice for an employer “to refuse to bargain collectively with the representatives of his employees, subject to the provisions of section 159 (a) of this title.” Section 159 (a) provides that “ [representatives designated or selected for the purposes of collective bargaining by the majority of the employees in a unit appropriate for such purposes, shall be the exclusive representatives of all the employees in such unit for the purposes of collective bargaining . . . .” Because the Act itself imposes a duty to bargain with the representative of a majority of the employees in an appropriate unit, the initial issue before the Board was whether the charging union was such a bargaining representative.
  The trial examiner first found that the unit designated by the regional director was an appropriate unit for bargaining. The unit found appropriate was defined as “[a] 11 full-time and regular part-time employees of [Burns] performing plant protection duties as determined in Section 9 (b) (3) of the [National Labor Relations] Act at Lockheed, Ontario International Airport; excluding office clerical employees, professional employees, supervisors, and all other employees as defined in the Act.” This determination was affirmed by the Board, accepted by the Court of Appeals, and is not at issue here because pretermitted by our limited grant of certiorari.
  The trial examiner then found, inter alia, that Burns “had in its employ a majority of Wackenhut’s former employees,” and that these employees had already expressed their choice of a bargaining representative in an election held a short time before. Burns was therefore held to have a duty to bargain, which arose when it selected as its work force the employees of the previous employer to perform the same tasks at the same place they had worked in the past.
  The Board, without revision, accepted the trial examiner’s findings and conclusions with respect to the duty to bargain, and we see no basis for setting them aside. In an election held but a few months before, the union had been designated bargaining agent for the employees in the unit and a majority of these employees had been hired by Burns for work in the identical unit. It is undisputed that Burns knew all the relevant facts in this regard and was aware of the certification and of the existence of a collective-bargaining contract. In these circumstances, it was not unreasonable for the Board to conclude that the union certified to represent all employees in the unit still represented a majority of the employees and that Burns could not reasonably have entertained a good-faith doubt about that fact. Burns’ obligation to bargain with the union over terms and conditions of employment stemmed from its hiring of Wackenhut’s employees and from the recent election and Board certification. It has been consistently held that a mere change of employers or of ownership in the employing industry is not such an “unusual circumstance” as to affect the force of the Board's certification within the normal operative period if a majority of employees after the change of ownership or management were employed by the preceding employer. NLRB v. Downtown Bakery Corp., 330 F. 2d 921, 925 (CA6 1964); NLRB v. McFarland, 306 F. 2d 219, 221 (CA10 1962); NLRB v. Auto Ventshade, Inc., 276 F. 2d 303, 307 (CA5 1960) ; NLRB v. Lander Shoe Corp., 211 F. 2d 284, 286 (CA1 1954); NLRB v. Armato, 199 F. 2d 800, 803 (CA7 1952) ; South Carolina Granite Co., 58 N. L. R. B. 1448, 1463-1464 (1944), enforced sub nom. NLRB v. Blair Quarries, Inc., 152 F. 2d 25 (CA4 1945); Northwest Glove Co., 74 N. L. R. B. 1697, 1700 (1947); Johnson Ready Mix Co., 142 N. L. R. B. 437, 442 (1963).
  It goes without saying, of course, that Burns was not entitled to upset what it should have accepted as an established union majority by soliciting representation cards for another union and thereby committing the unfair labor practice of which it was found guilty by the Board. That holding was not challenged here and makes it imperative that the situation be viewed as it was when Burns hired its employees for the guard unit, a majority of whom were represented by a Board-certified union. See NLRB v. Gissel Packing Co., 395 U. S. 575, 609, 610-616 (1969).
  It would be a wholly different case if the Board had determined that because Burns’ operational structure and practices differed from those of Wackenhut, the Lockheed bargaining unit was no longer an appropriate one. Likewise, it would be different if Burns had not hired employees already represented by a union certified as a bargaining agent, and the Board recognized as much at oral argument. But where the bargaining' unit remains unchanged and a majority of the employees hired by the new employer are represented by a recently certified bargaining agent there is little basis for faulting the Board’s implementation of the express mandates of § 8 (a) (5) and § 9 (a) by ordering the employer to bargain with the incumbent union. This is the view of several courts of appeals and we agree with those courts. NLRB v. Zayre Corp., 424 F. 2d 1159, 1162 (CA5 1970) ; Tom-A-Hawk Transit, Inc. v. NLRB, 419 F. 2d 1025, 1026-1027 (CA7 1969); S. S. Kresge Co. v. NLRB, 416 F. 2d 1225, 1234 (CA6 1969); NLRB v. McFarland, 306 F. 2d, at 220.
  Ill
  It does not follow, however, from Burns’ duty to bargain that it was bound to observe the substantive terms of the collective-bargaining contract the union had negotiated with Wackenhut and to which Burns had in no way agreed. Section 8 (d) of the Act expressly provides that the existence of such bargaining obligation “does not compel either party to agree to a proposal or require the making of a concession." Congress has consistently declined to interfere with free collective bargaining and has preferred that device, or voluntary arbitration, to the imposition of compulsory terms as a means of avoiding or terminating labor disputes. In its report accompanying the 1935 Act, the Senate Committee on Education and Labor stated:
  “The committee wishes to dispel any possible false impression that this bill is designed to compel the making of agreements or to permit governmental supervision of their terms. It must be stressed that the duty to bargain collectively does not carry with it the duty to reach an agreement, because the essence of collective bargaining is that either party shall be free to decide whether proposals made to it are satisfactory.” S. Rep. No. 573, 74th Cong., 1st Sess., 12 (1935).
  This Court immediately noted this fundamental theme of the legislation: “[The Act] does not compel any agreement whatever. . . . The theory of the Act is that free opportunity for negotiation with accredited representatives of employees is likely to promote industrial peace and may bring about the adjustments and agreements which the Act in itself does not attempt to compel.” NLRB v. Jones & Laughlin Steel Corp., 301 U. S. 1, 45 (1937). See also NLRB v. American National Insurance Co., 343 U. S. 395, 401-02 (1952); Teamsters Local 857 v. NLRB, 365 U. S. 667, 676-677 (1961).
  Section 8 (d), 29 U. S. C. § 158 (d), made this policy an express statutory mandate, and was enacted in 1947 because Congress feared that “the present Board has gone very far, in the guise of determining whether or not employers had bargained in good faith, in setting itself up as the judge of what concessions an employer must make and of the proposals and counterproposals that he may or may not make. . . . [UJnless Congress writes into the law guides for the Board to follow, the Board may attempt to carry this process still further and seek to control more and more the terms of collective bargaining agreements.” H. R. Rep. No. 245, 80th Cong., 1st Sess., 19-20 (1947).
  This history was reviewed in detail and given controlling effect in H. K. Porter Co. v. NLRB, 397 U. S. 99 (1970). There this Court, while agreeing that the employer violated §8 (a)(5) by adamantly refusing to agree to a dues checkoff, intending thereby to frustrate the consummation of any bargaining agreement, held that the Board had erred in ordering the employer to agree to such a provision:
  “[W]hile the Board does have power ... to require employers and employees to negotiate, it is without power to compel a company or a union to agree to any substantive contractual provision of a collective-bargaining agreement.
  “It would be anomalous indeed to hold that while § 8 (d) prohibits the Board from relying on a refusal to agree as the sole evidence of bad-faith bargaining, the Act permits the Board to compel agreement in that same dispute. The Board’s remedial powers under § 10 of the Act are broad, but they are limited to carrying out the policies of the Act itself. One of these fundamental policies is freedom of contract.” 397 U. S., at 102, 108 (citations omitted).
  These considerations, evident from the explicit language and legislative history of the labor laws, underlay the Board’s prior decisions, which until now have consistently held that, although successor employers may be bound to recognize and bargain with the union, they are not bound by the substantive provisions of a collective-bargaining contract negotiated by their predecessors but not agreed to or assumed by them. Rohlik, Inc., 145 N. L. R. B. 1236, 1242 n. 15 (1964); General Extrusion Co., 121 N. L. R. B. 1165, 1168 (1958); Jolly Giant Dumber Co., 114 N. L. R. B. 413, 414 (1955); Slater System Maryland, Inc., 134 N. L. R. B. 865, 866 (1961); Matter of ILWU (Juneau Spruce), 82 N. L. R. B. 650, 658-659 (1949), enforced, 189 F. 2d 177 (CA9 1951), aff’d on other grounds, 342 U. S. 237 (1952). As the Court of Appeals said in this case, “In none of the previous successorship cases has the Board ever reached that result. The successor has always been held merely to have the duty of bargaining with his predecessor’s union.” 441 F. 2d, at 915.
  The Board, however, has now departed from this view and argues that the same policies that mandate a continuity of bargaining obligation also require that successor employers be bound to the terms of a predecessor’s collective-bargaining contract. It asserts that the stability of labor relations will be jeopardized and that employees will face uncertainty and a gap in the bargained-for terms and conditions of employment, as well as the possible loss of advantages gained by prior negotiations, unless the new employer is held to have assumed, as a matter of federal labor law, the obligations under the contract entered into by the former employer. Recognizing that under normal contract principles a party would not be bound to a contract in the absence of consent, the Board notes that in John Wiley & Sons, Inc. v. Livingston, 376 U. S. 543, 550 (1964), the Court declared that “a collective bargaining agreement is not an ordinary contract” but is, rather, an outline of the common law of a particular plant or industry. The Court held in Wiley that although the predecessor employer which had signed a collective-bargaining contract with the union had disappeared by merger with the successor, the union could compel the successor to arbitrate the extent to which the successor was obligated under the collective-bargaining agreement. The Board contends that the same factors that the Court emphasized in Wiley, the peaceful settlement of industrial conflicts and “protection [of] the employees [against] a sudden change in the employment relationship,” id., at 549, require that Burns be treated under the collective-bargaining contract exactly as Wackenhut would have been if it had continued protecting the Lockheed plant.
  We do not find Wiley controlling in the circumstances here. Wiley arose in the context of a § 301 suit to compel arbitration, not in the context of an unfair labor practice proceeding where the Board is expressly limited by the provisions of §8(d). That decision emphasized “[t]he preference of national labor policy for arbitration as a substitute for tests of strength before contending forces” and held only that the agreement to arbitrate, “construed in the context of a national labor policy,” survived the merger and left to the arbitrator, subject to judicial review, the ultimate question of the extent to which, if any, the surviving company was bound by other provisions of the contract. Id., at 549, 551.
  Wiley’s limited accommodation between the legislative endorsement of freedom of contract and the judicial preference for peaceful arbitral settlement of labor disputes does not warrant the Board’s holding that the employer commits an unfair labor practice unless he honors the substantive terms of the pre-existing contract. The present case does not involve a § 301 suit; nor does it involve the duty to arbitrate. Rather, the claim is that Burns must be held bound by the contract executed by Wackenhut, whether Burns has agreed to it or not and even though Burns made it perfectly clear that it had no intention of assuming that contract. Wiley suggests no such open-ended obligation. Its narrower holding dealt with a merger occurring against a background of state law that embodied the general rule that in merger situations the surviving corporation is liable for the obligations of the disappearing corporation. See N. Y. Stock Corp. Law §90 (1951); 15 W. Fletcher, Private Corporations § 7121 (1961 rev. ed.). Here there was no merger or sale of assets, and there were no dealings whatsoever between Wackenhut and Burns. On the contrary, they were competitors for the same work, each bidding for the service contract at Lockheed. Burns purchased nothing from Wackenhut and became liable for none of its financial obligations. Burns merely hired enough of Wackenhut’s employees to require it to bargain with the union as commanded by § 8 (a)(5) and § 9 (a). But this consideration is a wholly insufficient basis for implying either in fact or in law that Burns had agreed or must be held to have agreed to honor Wackenhut’s collective-bargaining contract.
  We agree with the Court of Appeals that the Board failed to heed the admonitions of the H. K. Porter case. Preventing industrial strife is an important aim of federal labor legislation, but Congress has not chosen to make the bargaining freedom of employers and unions totally subordinate to this goal. When a bargaining impasse is reached, strikes and lockouts may occur. This bargaining freedom means both that parties need not make any concessions as a result of Government compulsion and that they are free from having contract provisions imposed upon them against their will. Here, Burns had notice of the existence of the Wackenhut collective-bargaining contract, but it did not consent to be bound by it. The source of its duty to bargain with the union is not the collective-bargaining contract but the fact that it voluntarily took over a bargaining unit that was largely intact and that had been certified within the past year. Nothing in its actions, however, indicated that Burns was assuming the obligations of the contract, and “allowing the Board to compel agreement when the parties themselves are unable to agree would violate the fundamental premise on which the Act is based — private bargaining under governmental supervision of the procedure alone, without any official compulsion over the actual terms of the contract.” H. K. Porter Co. v. NLRB, 397 U. S., at 108.
  We also agree with the Court of Appeals that holding either the union or the new employer bound to the substantive terms of an old collective-bargaining contract may result in serious inequities. A potential employer may be willing to take over a moribund business only if he can make changes in corporate structure, composition of the labor force, work location, task assignment, and nature of supervision. Saddling such an employer with the terms and conditions of employment contained in the old collective-bargaining contract may make these changes impossible and may discourage and inhibit the transfer of capital. On the other hand, a union may have made concessions to a small or failing employer that it would be unwilling to make to a large or economically successful firm. The congressional policy manifest in the Act is to enable the parties to negotiate for any protection either deems appropriate, but to allow the balance of bargaining advantage to be set by economic power realities. Strife is bound to occur if the concessions that must be honored do not correspond to the relative economic strength of the parties.
  The Board’s position would also raise new problems, for the successor employer would be circumscribed in exactly the same way as the predecessor under the collective-bargaining contract. It would seemingly follow that employees of the predecessor would be deemed employees of the successor, dischargeable only in accordance with provisions of the contract and subject to the grievance and arbitration provisions thereof. Burns would not have been free to replace Wackenhut’s guards with its own except as the contract permitted. Given the continuity of employment relationship, the pre-existing contract’s provisions with respect to wages, seniority-rights, vacation privileges, pension and retirement fund benefits, job security provisions, work assignments and the like would devolve on the successor. Nor would the union commit a § 8 (b) (3) unfair labor practice if it refused to bargain for a modification of the agreement effective prior to the expiration date of the agreement. A successor employer might also be deemed to have inherited its predecessor’s pre-existing contractual obligations to the union that had accrued under past contracts and that had not been discharged when the business was transferred. “[A] successor may well acquire more liabilities as a result of Burns than appear on the face of a contract.” Finally, a successor will be bound to observe the contract despite good-faith doubts about the union’s majority during the time that the contract is a bar to another representation election, Ranch-Way, Inc, 183 N. L. R. B. No. 116 (1970). For the above reasons, the Board itself has expressed doubts as to the general applicability of its Burns rule.
  In many cases, of course, successor employers will find it advantageous not only to recognize and bargain with the union but also to observe the pre-existing contract rather than to face uncertainty and turmoil. Also, in a variety of circumstances involving a merger, stock acquisition, reorganization, or assets purchase, the Board might properly find as a matter of fact that the successor had assumed the obligations under the old contract. Cf. Oilfield Maintenance Co., 142 N. L. R. B. 1384 (1963). Such a duty does not, however, ensue as a matter of law from the mere fact than an employer is doing the same work in the same place with the same employees as his predecessor, as the Board had recognized until its decision in the instant case. See cases cited supra, at 284. We accordingly set aside the Board’s finding of a § 8 (a)(5) unfair labor practice insofar as it rested on a conclusion that Burns was required to but did not honor the collective-bargaining contract executed by Wackenhut.
  IV
  It therefore follows that the Board’s order requiring Burns to “give retroactive effect to all the clauses of said [Wackenhut] contract and, with interest of 6 percent, make whole its employees for any losses suffered by reason of Respondent’s [Burns’] refusal to honor, adopt and enforce said contract” must be set aside. We note that the regional director’s charge instituting this case asserted that “[o]n or about July 1, 1967, Respondent [Burns] unilaterally changed existing wage rates, hours of employment, overtime wage rates, differentials for swing shift and graveyard shift, and other terms and conditions of employment of the employees in the appropriate unit . . . ,” App. 113, and that the Board’s opinion stated that “[t]he obligation to bargain imposed on a successor-employer includes the negative injunction to refrain from unilaterally changing wages and other benefits established by a prior collective-bargaining agreement even though that agreement had expired. In this respect, the successor-employer’s obligations are the same as those imposed upon employers generally during the period between collective-bargaining agreements.” App. 8-9. This statement by the Board is consistent with its prior and subsequent cases that hold that whether or not a successor employer is bound by its predecessor’s contract, it must not institute terms and conditions of employment different from those provided in its predecessor’s contract, at least without first bargaining with the employees’ representative. Overnite Transportation Co., 157 N. L. R. B. 1185 (1966), enforced sub nom. Overnite Transportation Co. v. NLRB, 372 F. 2d 765 (CA4), cert. denied, 389 U. S. 838 (1967); Valleydale Packers, Inc., 162 N. L. R. B. 1486 (1967), enforced sub nom. NLRB v. Vaileydale Packers, Inc., 402 F. 2d 768 (CA5 1968); Michaud Bus Lines, Inc., 171 N. L. R. B. 193 (1968); Emerald Maintenance, Inc., 188 N. L. R. B. No. 139 (1971). Thus, if Burns, without bargaining to impasse with the union, had paid its employees on and after July 1 at a rate lower than Wackenhut had paid under its contract, or otherwise provided terms and conditions of employment different from those provided in the Wackenhut collective-bargaining agreement, under the Board’s view, Burns would have committed a § 8 (a) (5) unfair labor practice and would have been subject to an order to restore to employees what they had lost by this so-called unilateral change. See Overnite Transportation Co., supra; Emerald Maintenance, Inc., supra.
  Although Burns had an obligation to bargain with the union concerning wages and other conditions of employment when the union requested it to do so, this case is not like a §8 (a)(5) violation where an employer unilaterally changes a condition of employment without consulting a bargaining representative. It is difficult to understand how Burns could be said to have changed unilaterally any pre-existing term or condition of employment without bargaining when it had no previous relationship whatsover to the bargaining unit and, prior to July 1, no outstanding terms and conditions of employment from which a change could be inferred. The terms on which Burns hired employees for service after July 1 may have differed from the terms extended by Wackenhut and required by the collective-bargaining contract; but it does not follow that Burns changed its terms and conditions of employment when it specified the initial basis on which employees were hired on July 1.
  Although a successor employer is ordinarily free to set initial terms on which it will hire the employees of a predecessor, there will be instances in which it is perfectly clear that the new employer plans to retain all of the employees in the unit and in which it will be appropriate to have him initially consult with the employees’ bargaining representative before he fixes terms. In other situations, however, it may not be clear until the successor employer has hired his full complement of employees that he has a duty to bargain with a union, since it will not be evident until then that the bargaining representative represents a majority of the employees in the unit as required by § 9 (a) of the Act, 29 U. S. C. § 159 (a). Here, for example, Burns’ obligation to bargain with the union did not mature until it had selected its force of guards late in June. The Board quite properly found that Burns refused to bargain on July 12 when it rejected the overtures of the union. It is true that the wages it paid when it began protecting the Lockheed plant on July 1 differed from those specified in the Wackenhut collective-bargaining agreement, but there is no evidence that Burns ever unilaterally changed the terms and conditions of employment it had offered to potential employees in June after its obligation to bargain with the union became apparent. If the union had made a request to bargain after Burns had completed its hiring and if Burns had negotiated in good faith and had made offers to the union which the union rejected, Burns could have unilaterally initiated such proposals as the opening terms and conditions of employment on July 1 without committing an unfair labor practice. Cf. NLRB v. Katz, 369 U. S. 736, 745 n. 12 (1962); NLRB v. Fitzgerald Mills Corp., 313 F. 2d 260, 272-273 (CA2) cert. denied, 375 U. S. 834 (1963); NLRB v. Southern Coach & Body Co., 336 F. 2d 214, 217 (CA5 1964). The Board’s order requiring Burns to make whole its employees for any losses suffered by reason of Burns’ refusal to honor and enforce the contract, cannot therefore be sustained on the ground that Burns unilaterally changed existing terms and conditions of employment, thereby committing an unfair labor practice which required monetary restitution in these circumstances.
  Affirmed.
  A Burns executive later admitted in the unfair-labor-practice proceeding that Burns was aware of the union’s status, the unit certification, and the collective-bargaining contract after the May 15 meeting. App. 105.
  In regard to this latter finding, the Board stated:
  “The question before us thus narrows to whether the national labor policy embodied in the Act requires the successor-employer to take over and honor a collective-bargaining agreement negotiated on behalf of the employing enterprise by the predecessor. We hold that, absent unusual circumstances, the Act imposes such an obligation.
  “We find, therefore, that Burns is bound to that contract as if it were a signatory thereto, and that its failure to maintain the contract in effect is violative of Sections 8 (d) and 8 (a) (5) of the Act.”
  Cf. § 9 (c) (3) of the NLRA, 29 U. S. C. § 159 (c) (3), which provides that “[n]o election shall be directed in any bargaining unit or any subdivision within which in the preceding twelve-month period, a valid election shall have been held.” See NLRB v. Gissel Packing Co., 395 U. S. 575, 599 n. 14 (1969).
  Where an employer remains the same, a Board certification carries with it an almost conclusive presumption that the majority representative status of the union continues for a reasonable time, usually a year. See Brooks v. NLRB, 348 U. S. 96, 98-99 (1954). After this period, there is a rebuttable presumption of majority representation. Celanese Corp. of America, 95 N. L. R. B. 664, 672 (1951). If there is a change of employers, however, and an almost complete turnover of employees, the certification may not bar a challenge if the successor employer is not bound by the collective-bargaining contract, particularly if the new employees are represented by another union or if the old unit is ruled an accretion to another unit. Cf. McGuire v. Humble Oil & Refining Co., 355 F. 2d 352 (CA2), cert. denied, 384 U. S. 988 (1966). See n. 5, infra.
  The Court of Appeals was unimpressed with the asserted differences between Burns’ and Wackenhut’s operations: “All of the important factors which the Board has used and the courts have approved are present in the instant case: ‘continuation of the same types of product lines, departmental organization, employee identity and job functions.’. . . Both Burns and Wackenhut are nationwide organizations; both performed the identical services at the same facility; although Burns used its own supervisors, their functions and responsibilities were similar to those performed by their predecessors; and finally, and perhaps most significantly, Burns commenced performance of the contract with 27 former Wackenhut employees out of its total complement of 42.” 441 F. 2d 911, 915 (1971) (citation omitted). Although the labor policies of the two companies differed somewhat, the Board’s determination that the bargaining unit remained appropriate after the changeover meant that Burns would face essentially the same labor relations environment as Wackenhut: it would confront the same union representing most of the same employees in the same unit.
  The Board has never held that the National Labor Relations Act itself requires that an employer who submits the winning bid for a service contract or who purchases the assets of a business be obligated to hire all of the employees of the predecessor though it is possible that such an obligation might be assumed by the employer. But cf. Chemrock Corp., 151 N. L. R. B. 1074 (1965). However, an employer who declines to hire employees solely because they are members of a union commits a § 8 (a) (3) unfair labor practice. See K. B. & J. Young’s Super Markets, Inc. v. NLRB, 377 F. 2d 463 (CA9), cert. denied, 389 U. S. 841 (1967); NLRB v. New England Tank Industries, Inc., 302 F. 2d 273 (CA1), cert. denied, 371 U. S. 875 (1962); Piasecki Aircraft Corp. v. NLRB, 280 F. 2d 575 (CA3 1960), cert. denied, 364 U. S. 933 (1961); Tri State Maintenance Corp., 167 N. L. R. B. 933 (1967), enforced with mod. sub nom. Tri State Maintenance Corp. v. NLRB, 132 U. S. App. D. C. 368, 408 F. 2d 171 (1968). Further restrictions on the successor employer’s choice of employees would seem to follow from the Board’s instant decision that the employer must honor the preexisting collective-bargaining contract. See infra, at 288-290.
  “Q. But [counsel for the Union], when he argued, said that even if [Burns] hadn’t taken over any [employees of Wackenhut], even if they hadn’t taken over a single employee, the legal situation would be the same.
  “Mr. Come [for the NLRB]. We do not go that far. We don’t think that you have to go that far in-
  “Q.’Do you think it has to be a majority?
  “Mr. Come. I wouldn’t say that it has to be a majority, I think it has to be a substantial number. It has to be enough to give you a continuity of employment conditions in the bargaining unit.” Tr. of Oral Arg. 64^65.
  Two exceptions to this general reluctance to interfere with free collective bargaining are the imposition of compulsory arbitration during wartime, Exec. Order No. 9017 (1942), and, on occasion, in the railroad industry, 77 Stat. 132, 81 Stat. 122. Congress has consistently rejected compulsory arbitration even as a remedy for “national emergency” disputes, however. See Goldberg, The Labor Law Obligations of a Successor Employer, 63 Nw. U. L. Rev. 735, 742-743 (1969).
  When the union that has signed a collective-bargaining contract is decertified, the succeeding union certified by the Board is not bound by the prior contract, need not administer it, and may demand negotiations for a new contract, even if the terms of the old contract have not yet expired. American Seating Co., 106 N. L. R. B. 250 (1953); Farmbest, Inc., 154 N. L. R. B. 1421, 1453-1454 (1965),, enf. with mod. sub nom. Farmbest, Inc. v. NLRB, 370 F. 2d 1015 (CA8 1967); see also Modine Mfg. Co. v. International Association of Machinists, 216 F. 2d 326 (CA6 1954). The Board has declined to overturn its “long standing” American Seating rule after Burns. General Dynamics Corp., 184 N. L. R. B. No. 71 (1970).
  The vast majority of collective-bargaining agreements specify the procedures to be used in choosing employees for available jobs, and approximately 92% of all such contracts place some limitations on the right to discharge. Collective Bargaining Negotiations and Contracts §§40.T, 60:11 (BNA 1971). Under the Board’s theory, if a successor refused to hire or fired any of the predecessor’s employees without going through applicable grievance procedures, it might be guilty of a § 8(a) (5) refusal to bargain. See NLRB v. Strong, 393 U. S. 357, 359 (1969); NLRB v. Huttig Sash & Door Co., 377 F. 2d 964, 968-969 (CA8 1967).
  Section 8 (d) of the Act provides, in part:
  “[W]here there is in effect a collective-bargaining contract covering employees in an industry affecting commerce, the duty to bargain collectively shall also mean that no party to such contract shall terminate or modify such contract, unless the party desiring such termination or modification—
  “(1) serves a written notice upon the other party to the contract of the proposed termination or modification sixty days prior to the expiration date thereof, or in the event such contract contains no expiration date, sixty days prior to the time it is proposed to make such termination or modification;
  “(2) offers to meet and confer with the other party for the purpose of negotiating a new contract or a contract containing the proposed modifications;
  “(3) notifies the Federal Mediation and Conciliation Service within thirty days after such notice of the existence of a dispute, and simultaneously therewith notifies any State or Territorial agency established to mediate and conciliate disputes within the State or Territory where the dispute occurred, provided no agreement has been reached by that time; and
  “(4) continues in full force and effect, without resorting to strike or lock-out, all the terms and conditions of the existing contract for a period of sixty days after such notice is given or until the expiration date of such contract, whichever occurs later:
  “The duties imposed upon employers, employees, and labor organizations by paragraphs (2)-(4) of this subsection . . . shall not be construed as requiring either party to discuss or agree to any modification of the terms and conditions contained in a contract for a fixed period, if such modification is to become effective before such terms and conditions can be reopened under the provisions of the contract.” 29 U. S. C. § 158 (d).
  Doppelt, Successor Companies: The NLRB Limits the Options— and Raises Some Problems, 20 DePaul L. Rev. 176, 191 (1971).
  The Board imposes this contract-bar rule for the term of a collective bargaining of “reasonable duration,” a period the Board now defines as three years. General Cable Corp., 139 N. L. R. B. 1123 (1962). Also during this time, an employer cannot use doubt about a union’s majority as a defense to a refusal-to-bargain charge. Oilfield Maintenance Co., 142 N. L. R. B. 1384, 1387 (1963); Hexton Furniture Co., 111 N. L. R. B. 342 (1955). Prior to Burns, the Board had held that a successor was barred by the contract of the predecessor from requesting a representation election during the term of the contract only if it had assumed the contract. Jolly Giant Lumber Co., 114 N. L. R. B. 413 (1955); General Extrusion Co., 121 N. L. R. B. 1165 (1958); MV Dominator, 162 N. L. R. B. 1514 (1967). Moreover, such assumption had to be by an express written agreement. American Concrete Pipe of Hawaii, Inc., 128 N. L. R. B. 720 (1960). The Board had also permitted a non-assuming successor to raise a good-faith doubt as to the union’s majority as a defense to a refusal-to-bargain charge during the term of the old contract. Randolph Rubber Co., 152 N. L. R. B. 496 (1965); Mitchell Standard Corp., 140 N. L. R. B. 496 (1963).
  Emerald Maintenance, Inc., 188 N. L. R. B. No. 139 (1971). Emerald involved a civilian contractor who undertook to provide certain maintenance services at an Air Force base. During the preceding year, the same services had been performed by two other companies whose employees were represented by a union that had negotiated collective-bargaining agreements that had not yet expired. The employer performed the work with substantially the same employee complement as had its predecessors. The Board held that the employer had a duty to recognize and bargain with the union but could not agree with the trial examiner that the employer was bound by the provisions of the contract, emphasizing in this respect the impact of the Service Contract Act of 1965, 79 Stat. 1034. The case was considered as presenting unusual circumstances justifying an exception to the Burns rule; the Board noted that “[t]his case suggests the hazards of enforcing the contracts of one employer against a successor where annual rebidding normally produces annual changes in contractor identity. These circumstances might encourage less arm’s-length collective bargaining whenever the employer had reason to expect that it would not be awarded the next succeeding annual service contract.” An amicus strongly contends that the Emerald rule is inconsistent with Burns and is based on a misreading of the legislative history of the Service Contract Act of 1965. Brief for AFL-CIO as Amicus Curiae 23 n. 2.
  In its entirety, the Board’s order required Burns to:
  “1. Cease and desist from:
  “(a) Refusing to bargain collectively, upon request, with the Union as the exclusive bargaining representative of its employees in the above-described unit.
  “(b) Refusing to adopt, honor and enforce its contract with the Union, as successor of Wackenhut.
  “(c) Assisting or recognizing AFG as the representative of its employees for the purposes of collective bargaining, unless and until said labor organization shall have been certified as the exclusive bargaining representative of said employees in an appropriate unit.
  “(d) Interfering with representation of its employees through labor organizations of their own choosing.
  “(e) In any like or related manner interfering with, restraining, or coercing its employees in the exercise of their rights to join or assist the Union or otherwise engage in activities protected by the Act.
  “2. Take the following affirmative action which is necessary to effectuate the policies of the Act:
  “(a) Withdraw and withhold recognition from AFG until or unless it is certified as bargaining representative of Respondent’s employees in an appropriate unit.
  “(b) Bargain collectively, upon request, with the Union and, if any understanding is reached, embody such understanding in a signed agreement.
  “(c) Honor, adopt and enforce the contract between.Respondent, as successor to Wackenhut, and the Union and give retroactive effect to all the clauses of said contract and, with interest of 6 percent, make whole its employees for any losses suffered by reason of Respondent’s refusal to honor, adopt and enforce said contract.
  “(d) Post at its Lockheed, Ontario, California, operations copies of the notice attached hereto as ‘Appendix.’ Copies of said notice, to be furnished by the Regional Director for Region 31, shall after being signed by Respondent’s authorized representative, be posted by it immediately upon receipt thereof and be maintained by it for a period of 60 consecutive days in conspicuous places, including all places where notices to employees are customarily posted. Reasonable steps shall be taken to insure that such notices are not altered, defaced or covered by any other material.
  “(e) Notify said Regional Director, in writing, within 20 days from the date of the receipt of this Recommended Order what steps Respondent has taken to comply herewith.”
  “While a broader unit might have been appropriate, I find a unit of guards limited to a single facility as an appropriate unit. Here, the certification was pursuant to a consent election agreement.” Trial Examiner’s decision, App. 22.