Patients who are unable to tolerate an oral antibiotic may be treated with ampicillin, ceftriaxone, or cefazolin administered intramuscularly or intravenously. For ampicillin-allergic patients who are unable to tolerate an oral agent, therapy is recommended with parenteral cefazolin, ceftriaxone, or clindamycin. Regimens for Respiratory Tract Procedures A variety of respiratory tract procedures reportedly cause tran- sient bacteremia with a wide array of microorganisms1; how- ever, no published data conclusively demonstrate a link between these procedures and IE. Antibiotic prophylaxis with a regimen listed in Table 5 is reasonable (Class IIa, LOE C) for patients with the conditions listed in Table 3 who undergo an invasive procedure of the respiratory tract that involves incision or biopsy of the respiratory mucosa, such as tonsillectomy and adenoid- ectomy. We do not recommend antibiotic prophylaxis for bronchoscopy unless the procedure involves incision of the respiratory tract mucosa. For patients listed in Table 3 who undergo an invasive respiratory tract procedure to treat an established infection, such as drainage of an abscess or empy- ema, we recommend that the antibiotic regimen administered to these patients contain an agent active against viridans group streptococci (Table 5). If the infection is known or suspected to be caused by Staphylococcus aureus, the regimen should contain an agent active against S aureus, such as an antistaphylococcal penicillin or cephalosporin, or vancomycin in patients unable to tolerate a -lactam. Vancomycin should be administered if the infection is known or suspected to be caused by a methicillin- resistant strain of S aureus. Recommendations for GI or GU Tract Procedures Enterococci are part of the normal flora of the GI tract. These microorganisms may cause intra-abdominal infection or infection of the hepatobiliary system. 