opinion: Justice Stevens
  delivered the opinion of the Court.
  During their examination of a damaged package, the employees of a private freight carrier observed a white powdery substance, originally concealed within eight layers of wrappings. They summoned a federal agent, who removed a trace of the powder, subjected it to a chemical test and determined that it was cocaine. The question presented is whether the Fourth Amendment required the agent to obtain a warrant before he did so.
  The relevant facts are not in dispute. Early in the morning of May 1, 1981, a supervisor at the Minneapolis-St. Paul Airport Federal Express office asked the office manager to look at a package that had been damaged and torn by a forklift. They then opened the package in order to examine its contents pursuant to a written company policy regarding insurance claims.
  The container was an ordinary cardboard box wrapped in brown paper. Inside the box five or six pieces of crumpled newspaper covered a tube about 10 inches long; the tube was made of the silver tape used on basement ducts. The supervisor and office manager cut open the tube, and found a series of four zip-lock plastic bags, the outermost enclosing the other three and the innermost containing about six and a half ounces of white powder. When they observed the white powder in the innermost bag, they notified the Drug Enforcement Administration. Before the first DEA agent arrived, they replaced the plastic bags in the tube and put the tube and the newspapers back into the box.
  When the first federal agent arrived, the box, still wrapped in brown paper, but with a hole punched in its side and the top open, was placed on a desk. The agent saw that one end of the tube had been slit open; he removed the four plastic bags from the tube and saw the white powder. He then opened each of the four bags and removed a trace of the white substance with a knife blade. A field test made on the spot identified the substance as cocaine.
  In due course, other agents arrived, made a second field test, rewrapped the package, obtained a warrant to search the place to which it was addressed, executed the warrant, and arrested respondents. After they were indicted for the crime of possessing an illegal substance with intent to distribute, their motion to suppress the evidence on the ground that the warrant was the product of an illegal search and seizure was denied; they were tried and convicted, and appealed. The Court of Appeals reversed. 683 F. 2d 296 (CA8 1982). It held that the validity of the search warrant depended on the validity of the agents’ warrantless test of the white powder, that the testing constituted a significant expansion of the earlier private search, and that a warrant was required.
  As the Court of Appeals recognized, its decision conflicted with a decision of another Court of Appeals on comparable facts, United States v. Barry, 673 F. 2d 912 (CA6), cert. denied, 459 U. S. 927 (1982). For that reason, and because field tests play an important role in the enforcement of the narcotics laws, we granted certiorari, 460 U. S. 1021.
  r*H
  The first Clause of the Fourth Amendment provides that the “right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated . . . This text protects two types of expectations, one involving “searches,” the other “seizures.” A “search” occurs when an expectation of privacy that society is prepared to consider reasonable is infringed. A “seizure” of property occurs when there is some meaningful interference with an individual’s possessory interests in that property. This Court has also consistently construed this protection as proscribing only governmental action; it is wholly inapplicable “to a search or seizure, even an unreasonable one, effected by a private individual not acting as an agent of the Government or with the participation or knowledge of any governmental official.” Walter v. United States, 447 U. S. 649, 662 (1980) (Blackmun, J., dissenting).
  When the wrapped parcel involved in this case was delivered to the private freight carrier, it was unquestionably an “effect” -within the meaning of the Fourth Amendment. Letters and other sealed packages are in the general class of effects in which the public at large has a legitimate expectation of privacy; warrantless searches of such effects are presumptively unreasonable. Even when government agents may lawfully seize such a package to prevent loss or destruction of suspected contraband, the Fourth Amendment requires that they obtain a warrant before examining the contents of such a package. Such a warrantless search could not be characterized as reasonable simply because, after the official invasion of privacy occurred, contraband is discovered. Conversely, in this case the fact that agents of the private carrier independently opened the package and made an examination that might have been impermissible for a government agent cannot render otherwise reasonable official conduct unreasonable. The reasonableness of an official invasion of the citizen’s privacy must be appraised on the basis of the facts as they existed at the time that invasion occurred.
  The initial invasions of respondents’ package were occasioned by private action. Those invasions revealed that the package contained only one significant item, a suspicious looking tape tube. Cutting the end of the tube and extracting its contents revealed a suspicious looking plastic bag of white powder. Whether those invasions were accidental or deliberate, and whether they were reasonable or unreasonable, they did not violate the Fourth Amendment because of their private character.
  The additional invasions of respondents’ privacy by the Government agent must be tested by the degree to which they exceeded the scope of the private search. That standard was adopted by a majority of the Court in Walter v. United States, supra. In Walter a private party had opened a misdirected carton, found rolls of motion picture films that appeared to be contraband, and turned the carton over to the Federal Bureau of Investigation. Later, without obtaining a warrant, FBI agents obtained a projector and viewed the films. While there was no single opinion of the Court, a majority did agree on the appropriate analysis of a governmental search which follows on the heels of a private one. Two Justices took the position:
  “If a properly authorized official search is limited by the particular terms of its authorization, at least the same kind of strict limitation must be applied to any official use of a private party’s invasion of another person’s privacy. Even though some circumstances — for example, if the results of the private search are in plain view when materials are turned over to the Government— may justify the Government’s reexamination of the materials, surely the Government may not exceed the scope of the private search unless it has the right to make an independent search. In these cases, the private party had not actually viewed the films. Prior to the Government screening, one could only draw inferences about what was on the films. The projection of the films was a significant expansion of the search that had been conducted previously by a private party and therefore must be characterized as a separate search.” Id., at 657 (opinion of Stevens, J., joined by Stewart, J.) (footnote omitted).
  Four additional Justices, while disagreeing with this characterization of the scope of the private search, were also of the view that the legality of the governmental search must be tested by the scope of the antecedent private search.
  “ ‘Under these circumstances, since the L’Eggs employees so fully ascertained the nature of the films before contacting the authorities, we find that the FBI’s subsequent viewing of the movies on a projector did not “change the nature of the search” and was not an additional search subject to the warrant requirement.’” Id., at 663-664 (Blackmun, J., dissenting, joined by Burger, C. J., and Powell and Rehnquist, JJ.) (footnote omitted) (quoting United States v. Sanders, 592 P. 2d 788, 793-794 (CA5 1979) (case below in Walter)
  This standard follows from the analysis applicable when private parties reveal other kinds of private information to the authorities. It is well settled that when an individual reveals private information to another, he assumes the risk that his confidant will reveal that information to the authorities, and if that occurs the Fourth Amendment does not prohibit governmental use of that information. Once frustration of the original expectation of privacy occurs, the Fourth Amendment does not prohibit governmental use of the now nonprivate information: “This Court has held repeatedly that the Fourth Amendment does not prohibit the obtaining of information revealed to a third party and conveyed by him to Government authorities, even if the information is revealed on the assumption that it will be used only for a limited purpose and the confidence placed in a third party will not be betrayed.” United States v. Miller, 425 U. S. 435, 443 (1976). The Fourth Amendment is implicated only if the authorities use information with respect to which the expectation of privacy has not already been frustrated. In such a case the authorities have not relied on what is in effect a private search, and therefore presumptively violate the Fourth Amendment if they act without a warrant.
  In this case, the federal agents’ invasions of respondents’ privacy involved two steps: first, they removed the tube from the box, the plastic bags from the tube, and a trace of powder from the innermost bag; second, they made a chemical test of the powder. Although we ultimately conclude that both actions were reasonable for essentially the same reason, it is useful to discuss them separately.
  II
  When the first federal agent on the scene initially saw the package, he knew it contained nothing of significance except a tube containing plastic bags and, ultimately, white powder. It is not entirely clear that the powder was visible to him before he removed the tube from the box. Even if the white powder was not itself in “plain view” because it was still enclosed in so many containers and covered with papers, there was a virtual certainty that nothing else of significance was in the package and that a manual inspection of the tube and its contents would not tell him anything more than he already had been told. Respondents do not dispute that the Government could utilize the Federal Express employees’ testimony concerning the contents of the package. If that is the case, it hardly infringed respondents’ privacy for the agents to reexamine the contents of the open package by brushing aside a crumpled newspaper and picking up the tube. The advantage the Government gained thereby was merely avoiding the risk of a flaw in the employees’ recollection, rather than in further infringing respondents’ privacy. Protecting the risk of misdescription hardly enhances any legitimate privacy interest, and is not protected by the Fourth Amendment. Respondents could have no privacy interest in the contents of the package, since it remained unsealed and since the Federal Express employees had just examined the package and had, of their own accord, invited the federal agent to their offices for the express purpose of viewing its contents. The agent’s viewing of what a private party had freely made available for his inspection did not violate the Fourth Amendment. See Coolidge v. New Hampshire, 403 U. S. 443, 487-490 (1971); Burdeau v. McDowell, 256 U. S. 465, 475-476 (1921).
  Similarly, the removal of the plastic bags from the tube and the agent’s visual inspection of their contents enabled the agent to learn nothing that had not previously been learned during the private search. It infringed no legitimate expectation of privacy and hence was not a “search” within the meaning of the Fourth Amendment.
  While the agents’ assertion of dominion and control over the package and its contents did constitute a “seizure,” that seizure was not unreasonable. The fact that, prior to the field test, respondents’ privacy interest in the contents of the package had been largely compromised is highly relevant to the reasonableness of the agents’ conduct in this respect. The agents had already learned a great deal about the contents of the package from the Federal Express employees, all of which was consistent with what they could see. The package itself, which had previously been opened, remained unsealed, and the Federal Express employees had invited the agents to examine its contents. Under these circumstances, the package could no longer support any expectation of privacy; it was just like a balloon “the distinctive character [of which] spoke volumes as to its contents — particularly to the trained eye of the officer,” Texas v. Brown, 460 U. S. 730, 743 (1983) (plurality opinion); see also id,., at 746 (Powell, J., concurring in judgment); or the hypothetical gun case in Arkansas v. Sanders, 442 U. S. 753, 764-765, n. 13 (1979). Such containers may be seized, at least temporarily, without a warrant. Accordingly, since it was apparent that the tube and plastic bags contained contraband and little else, this warrantless seizure was reasonable, for it is well settled that it is constitutionally reasonable for law enforcement officials to seize “effects” that cannot support a justifiable expectation of privacy without a warrant, based on probable cause to believe they contain contraband.
  r-H I — l I — I
  The question remains whether the additional intrusion occasioned by the field test, which had not been conducted by the Federal Express employees and therefore exceeded the scope of the private search, was an unlawful “search” or “seizure” within the meaning of the Fourth Amendment.
  The field test at issue could disclose only one fact previously unknown to the agent — whether or not a suspicious white powder was cocaine. It could tell him nothing more, not even whether the substance was sugar or talcum powder. We must first determine whether this can be considered a “search” subject to the Fourth Amendment — did it infringe an expectation of privacy that society is prepared to consider reasonable?
  The concept of an interest in privacy that society is prepared to recognize as reasonable is, by its very nature, critically different from the mere expectation, however well justified, that certain facts will not come to the attention of the authorities. Indeed, this distinction underlies the rule that government may utilize information voluntarily disclosed to a governmental informant, despite the criminal’s reasonable expectation that his associates would not disclose confidential information to the authorities. See United States v. White, 401 U. S. 745, 751-752 (1971) (plurality opinion).
  A chemical test that merely discloses whether or not a particular substance is cocaine does not compromise any legitimate interest in privacy. This conclusion is not dependent on the result of any particular test. It is probably safe to assume that virtually all of the tests conducted under circumstances comparable to those disclosed by this record would result in a positive finding; in such cases, no legitimate interest has been compromised. But even if the results are negative — merely disclosing that the substance is something other than cocaine — such a result reveals nothing of special interest. Congress has decided — and there is no question about its power to do so — to treat the interest in “privately” possessing cocaine as illegitimate; thus governmental conduct that can reveal whether a substance is cocaine, and no other arguably “private” fact, compromises no legitimate privacy interest.
  This conclusion is dictated by United States v. Place, 462 U. S. 696 (1983), in which the Court held that subjecting luggage to a “sniff test” by a trained narcotics detection dog was not a “search” within the meaning of the Fourth Amendment:
  “A ‘canine sniff’ by a well-trained narcotics detection dog, however, does not require opening the luggage. It does not expose noncontraband items that otherwise would remain hidden from public view, as does, for example, an officer’s rummaging through the contents of the luggage. Thus, the manner in which information is obtained through this investigative technique is much less intrusive than a typical search. Moreover, the sniff discloses only the presence or absence of narcotics, a contraband item. Thus, despite the fact that the sniff tells the authorities something about the contents of the luggage, the information obtained is limited.” Id., at 707.
  Here, as in Place, the likelihood that official conduct of the kind disclosed by the récord will actually compromise any legitimate interest in privacy seems much too remote to characterize the testing as a search subject to the Fourth Amendment.
  We have concluded, in Part II, supra, that the initial “seizure” of the package and its contents was reasonable. Nevertheless, as Place also holds, a seizure lawful at its inception can nevertheless violate the Fourth Amendment because its manner of execution unreasonably infringes possessory interests protected by the the Fourth Amendment’s prohibition on “unreasonable seizures.” Here, the field test did affect respondents’ possessory interests protected by the Amendment, since by destroying a quantity of the powder it converted what had been only a temporary deprivation of pos-sessory interests into a permanent one. To assess the reasonableness of this conduct, “[w]e must balance the nature and quality of the intrusion on the individual’s Fourth Amendment interests against the importance of the governmental interests alleged to justify the intrusion.” 462 U. S., at 703.
  Applying this test, we conclude that the destruction of the powder during the course of the field test was reasonable. The law enforcement interests justifying the procedure were substantial; the suspicious nature of the material made it virtually certain that the substance tested was in fact contraband. Conversely, because only a trace amount of material was involved, the loss of which appears to have gone unnoticed by respondents, and since the property had already been lawfully detained, the “seizure” could, at most, have only a de minimis impact on any protected property interest. Cf. Cardwell v. Lewis, 417 U. S. 583, 591-592 (1974) (plurality opinion) (examination of automobile’s tires and taking of paint scrapings was a de minimis invasion of constitutional interests). Under these circumstances, the safeguards of a warrant would only minimally advance Fourth Amendment interests. This warrantless “seizure” was reasonable.
  In sum, the federal agents did not infringe any constitutionally protected privacy interest that had not already been frustrated as the result of private conduct. To the extent that a protected possessory interest was infringed, the infringement was de minimis and constitutionally reasonable. The judgment of the Court of Appeals is
  Reversed.
  As the test is described in the evidence, it involved the use of three test tubes. When a substance containing cocaine is placed in one test tube after another, it will cause liquids to take on a certain sequence of colors. Such a test discloses whether or not the substance is cocaine, but there is no evidence that it would identify any other substances.
  The Court of Appeals did not hold that the facts would not have justified the issuance of a warrant without reference to the test results; the court merely held that the facts recited in the warrant application, which relied almost entirely on the results of the field tests, would not support the issuance of the warrant if the field test was itself unlawful. “ ‘It is elementary that in passing on the validity of a warrant, the reviewing court may consider only information brought to the magistrate’s attention.’ ” Spinelli v. United States, 393 U. S. 410, 413, n. 3 (1969) (emphasis in original) (quoting Aguilar v. Texas, 378 U. S. 108, 109, n. 1 (1964)). See Illinois v. Gates, 462 U. S. 213, 238-239 (1983).
  See also People v. Adler, 50 N. Y. 2d 730, 409 N. E. 2d 888, cert. denied, 449 U. S. 1014 (1980); cf. United States v. Andrews, 618 F. 2d 646 (CA10) (upholding warrantless field test without discussion), cert. denied, 449 U. S. 824 (1980).
  See Illinois v. Andreas, 463 U. S. 765, 771 (1983); United States v. Knotts, 460 U. S. 276, 280-281 (1983); Smith v. Maryland, 442 U. S. 735, 739-741 (1979); Terry v. Ohio, 392 U. S. 1, 9 (1968).
  See United States v. Place, 462 U. S. 696 (1983); id., at 716 (Brennan, J., concurring in result); Texas v. Brown, 460 U. S. 730, 747-748 (1983) (Stevens, J., concurring in judgment); see also United States v. Chadwick, 433 U. S. 1, 13-14, n. 8 (1977); Hale v. Henkel, 201 U. S. 43, 76 (1906). While the concept of a “seizure” of property is not much discussed in our cases, this definition follows from our oft-repeated definition of the “seizure” of a person within the meaning of the Fourth Amendment— meaningful interference, however brief, with an individual’s freedom of movement. See Michigan v. Summers, 452 U. S. 692, 696 (1981); Reid v. Georgia, 448 U. S. 438, 440, n. (1980) (per curiam); United States v. Mendenhall, 446 U. S. 544, 551-554 (1980) (opinion of Stewart, J.); Brown v. Texas, 443 U. S. 47, 50 (1979); United Stales v. Brignoni-Ponce, 422 U. S. 873, 878 (1975); Cupp v. Murphy, 412 U. S. 291, 294-295 (1973); Davis v. Mississippi, 394 U. S. 721, 726-727 (1969); Terry v. Ohio, 392 U. S., at 16, 19, n. 16.
  See 447 U. S., at 656 (opinion of Stevens, J.); id., at 660-661 (White, J., concurring in part and concurring in judgment); United States v. Janis, 428 U. S. 433, 455-456, n. 31 (1976); Coolidge v. New Hampshire, 403 U. S. 443, 487-490 (1971); Burdeau v. McDowell, 256 U. S. 465 (1921).
  United States v. Chadwick, 433 U. S. 1, 10 (1977); United States v. Van Leeuwen, 397 U. S. 249, 251 (1970); Ex parte Jackson, 96 U. S. 727, 733 (1878); see also Walter, 447 U. S., at 654-655 (opinion of Stevens, J.).
  See, e. g., United States v. Place, 462 U. S., at 701; United States v. Ross, 456 U. S. 798, 809-812 (1982); Robbins v. California, 453 U. S. 420, 426 (1981) (plurality opinion); Arkansas v. Sanders, 442 U. S. 753, 762 (1979); United States v. Chadwick, 433 U. S., at 13, and n. 8; United States v. Van Leeuwen, supra. There is, of course, a well-recognized exception for customs searches; but that exception is not involved in this case.
  See Whiteley v. Warden, 401 U. S. 560, 567, n. 11 (1971); Wong Sun v. United States, 371 U. S. 471, 484 (1963); Rios v. United States, 364 U. S. 253, 261-262 (1960); Henry v. United States, 361 U. S. 98, 103 (1959); Miller v. United States, 357 U. S. 301, 312 (1958); United States v. Di Re, 332 U. S. 581, 595 (1948); Byars v. United States, 273 U. S. 28, 29 (1927).
  A post-trial affidavit indicates that an agent of Federal Express may have opened the package because he was suspicious about its contents, and not because of damage from a forklift. However, the lower courts found no governmental involvement in the private search, a finding not challenged by respondents. The affidavit thus is of no relevance to the issue we decide.
  See also 447 U. S., at 658-659 (footnotes omitted) (“The fact that the cartons were unexpectedly opened by a third party before the shipment was delivered to its intended consignee does not alter the consignor’s legitimate expectation of privacy. The private search merely frustrated that expectation in part. It did not simply strip the remaining unfrustrated portion of that expectation of all Fourth Amendment protection”).
  In Walter, a majority of the Court found a violation of the Fourth Amendment. For present purposes, the disagreement between the majority and the dissenters in that case with respect to the comparison between the private search and the official search is less significant than the agreement on the standard to be applied in evaluating the relationship between the two searches.
  See Smith v. Maryland, 442 U. S. 735, 743-744 (1979); United States v. White, 401 U. S. 745, 749-753 (1971) (plurality opinion); Osborn v. United States, 385 U. S. 323, 326-331 (1966); Hoffa v. United States, 385 U. S. 293, 300-303 (1966); Lewis v. United States, 385 U. S. 206 (1966); Lopez v. United States, 373 U. S. 427, 437-439 (1963); On Lee v. United States, 343 U. S. 747, 753-754 (1952). See also United States v. Henry, 447 U. S. 264, 272 (1980); United States v. Caceres, 440 U. S. 741, 744, 750-751 (1979).
  See Katz v. United States, 389 U. S. 347 (1967); Berger v. New York, 388 U. S. 41 (1967); Silverman v. United States, 365 U. S. 505 (1961).
  Daniel Stegemoller, the Federal Express office manager, testified at the suppression hearing that the white substance was not visible without reentering the package at the time the first agent arrived. App. 42-43, 58. As Justice White points out, the Magistrate found that the “tube was in plain view in the box and the bags with the white powder were visible from the end of the tube.” App. to Pet. for Cert. 18a. The bags were, however, only visible if one picked up the tube and peered inside through a small aperture; even then, what was visible was only the translucent bag that contained the white powder. The powder itself was barely visible, and surely was not so plainly in view that the agents did “no more than fail to avert their eyes,” post, at 130. In any event, respondents filed objections to the Magistrate’s report with the District Court. The District Court declined to resolve respondents’ objections, ruling that fact immaterial and assuming for purposes of its decision “that the newspaper in the box covered the gray tube and that neither the gray tube nor the contraband could be seen when the box was turned over to the. . . DEA agents.” App. to Pet. for Cert. 12a-13a. At trial, the federal agent first on the scene testified that the powder was not visible until after he pulled the plastic bags out of the tube. App. 71-72. Respondents continue to argue this case on the assumption that the Magistrate’s report is incorrect. Brief for Respondents 2-3. As our discussion will make clear, we agree with the Distriet Court that it does not matter whether the loose pieces of newspaper covered the tube at the time the agent first saw the box.
  See United States v. Caceres, 440 U. S., at 750-751; United States v. White, 401 U. S., at 749-753 (plurality opinion); Osborn v. United States, 385 U. S., at 326-331; On Lee v. United States, 343 U. S., at 753-754. For example, in Lopez v. United States, 373 U. S. 427 (1963), the Court wrote: “Stripped to its essentials, petitioner’s argument amounts to saying that he has a constitutional right to rely on possible flaws in the agent’s memory, or to challenge the agent’s credibility without being beset by corroborating evidence.... For no other argument can justify excluding an accurate version of a conversation that the agent could testify to from memory. We think the risk that petitioner took in offering a bribe to Davis fairly included the risk that the offer would be accurately reproduced in court. . . .” Id., at 439 (footnote omitted).
  We reject Justice White’s suggestion that this case is indistinguishable from one in which the police simply learn from a private party that a container contains contraband, seize it from its owner, and conduct a war-rantless search which, as Justice White properly observes, would be unconstitutional. Here, the Federal Express employees who were lawfully in possession of the package invited the agent to examine its contents; the governmental conduct was made possible only because private parties had compromised the integrity of this container. Justice White would have this case turn on the fortuity of whether the Federal Express employees placed the tube back into the box. But in the context of their previous examination of the package, their communication of what they had learned to the agent, and their offer to have the agent inspect it, that act surely could not create any privacy interest with respect to the package that would not otherwise exist. See Illinois v. Andreas, 463 U. S., at 771-772. Thus the precise character of the white powder’s visibility to the naked eye is far less significant than the facts that the container could no longer support any expectation of privacy, and that it was virtually certain that it contained nothing but contraband. Contrary to Justice White’s suggestion, we do not “sanctio[n] warrantless searches of closed or covered containers or packages whenever probable cause exists as a result of a prior private search.” Post, at 129. A container which can support a reasonable expectation of privacy may not be searched, even on probable cause, without a warrant. See United States v. Ross, 456 U. S., at 809-812; Robbins v. California, 453 U. S., at 426-427 (plurality opinion); Arkansas v. Sanders, 442 U. S., at 764-765; United States v. Chadwick, 433 U. S. 1 (1977).
  Both the Magistrate and the District Court found that the agents took custody of the package from Federal Express after they arrived. Although respondents had entrusted possession of the items to Federal Express, the decision by governmental authorities to exert dominion and control over the package for their own purposes clearly constituted a “seizure,” though not necessarily an unreasonable one. See United States v. Van Leeuwen, 397 U. S. 249 (1970). Indeed, this is one thing on which the entire Court appeared to agree in Walter v. United States, 447 U. S. 649 (1980).
  See also United States v. Ross, 456 U. S., at 822-823; Robbins v. California, 453 U. S., at 428 (plurality opinion).
  Respondents concede that the agents had probable cause to believe the package contained contraband. Therefore we need not decide whether the agents could have seized the package based on something less than probable cause. Some seizures can be justified by an articulable suspicion of criminal activity. See United States v. Place, 462 U. S. 696 (1983).
  See Place, 462 U. S., at 701-702; Texas v. Brown, 460 U. S., at 741-742 (plurality opinion); id., at 748 (Stevens, J., concurring in judgment); Payton v. New York, 445 U. S. 573, 587 (1980); G. M. Leasing Corp. v. United States, 429 U. S. 338, 354 (1977); Harris v. United States, 390 U. S. 234, 236 (1968) (per curiam).
  “Obviously, however, a ‘legitimate’ expectation of privacy by definition means more than a subjective expectation of not being discovered. A burglar plying his trade in a summer cabin during the off season may have a thoroughly justified subjective expectation of privacy, but it is not one which the law recognizes as ‘legitimate.’ His presence, in the words of Jones [v. United States, 362 U. S. 257, 267 (1960)], is ‘wrongful’; his expectation [of privacy] is not ‘one that society is prepared to recognize as “reasonable.” ’ Katz v. United States, 389 U. S., at 361 (Harlan, J., concurring). And it would, of course, be merely tautological to fall back on the notion that those expectations of privacy which are legitimate depend primarily on cases deciding exclusionary-rule issues in criminal cases. Legitimation of expectations of privacy by law must have a source outside of the Fourth Amendment, either by reference to concepts of real or personal property law or to understandings that are recognized and permitted by society.” Rakas v. Illinois, 439 U. S. 128, 143-144, n. 12 (1978). See also United States v. Knotts, 460 U. S. 276 (1983) (use of a beeper to track ear’s movements infringed no reasonable expectation of privacy); Smith v. Maryland, 442 U. S. 735 (1979) (use of a pen register to record phone numbers dialed infringed no reasonable expectation of privacy).
  See Loewy, The Fourth Amendment as a Device for Protecting the Innocent, 81 Mich. L. Rev. 1229 (1983). Our discussion, of course, is confined to possession of contraband. It is not necessarily the case that the purely “private” possession of an article that cannot be distributed in commerce is itself illegitimate. See Stanley v. Georgia, 394 U. S. 557 (1969).
  Respondents attempt to distinguish Place, arguing that it involved no physical invasion of Place’s effects, unlike the conduct at issue here. However, as the quotation makes clear, the reason this did not intrude upon any legitimate privacy interest was that the governmental conduct could reveal nothing about noncontraband items. That rationale is fully applicable here.
  In Place, the Court held that while the initial seizure of luggage for the purpose of subjecting it to a “dog sniff” test was reasonable, the seizure became unreasonable because its length unduly intruded upon constitutionally protected interests. See id., at 707-710.
  See, e. g., Michigan v. Long, 463 U. S. 1032, 1046-1047 (1983); Delaware v. Prouse, 440 U. S. 648, 654 (1979); United States v. Brignoni-Ponce, 422 U. S., at 878; Terry v. Ohio, 392 U. S., at 20-21; Camara v. Municipal Court, 387 U. S. 523, 536-537 (1967).
  In fact, respondents do not contend that the amount of material tested was large enough to make it possible for them to have detected its loss. The only description in the record of the amount of cocaine seized is that “[i]t was a trace amount.” App. 75.
  See Cupp v. Murphy, 412 U. S. 291, 296 (1973) (warrantless search and seizure limited to scraping suspect’s fingernails justified even when full search may not be). Cf. Place, 462 U. S., at 703-706 (approving brief warrantless seizure of luggage for purposes of “sniff test” based on its minimal intrusiveness and reasonable belief that the luggage contained contraband); United States v. Van Leeuwen, 397 U. S., at 252-253 (detention of package on reasonable suspicion was justified since detention infringed no “significant Fourth Amendment interest”). Of course, where more substantial invasions of constitutionally protected interests are involved, a warrantless search or seizure is unreasonable in the absence of exigent circumstances. See, e. g., Steagald v. United States, 451 U. S. 204 (1981); Payton v. New York, 445 U. S. 573 (1980); Dunaway v. New York, 442 U. S. 200 (1979); United States v. Chadwick, 433 U. S. 1 (1977). We do not suggest, however, that any seizure of a small amount of material is necessarily reasonable. An agent’s arbitrary decision to take the “white powder” he finds in a neighbor’s sugar bowl, or his medicine cabinet, and subject it to a field test for cocaine, might well work an unreasonable seizure.
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