589 isolates of anaerobic bacteria, including 127 isolates of Bacteroides fragilis, were tested against 23 antimicrobial agents. At their usually achievable serum levels, carbenicillin, ticarcillin, mezlocillin and piperacillin; cefoxitin, minocycline, doxycycline, clindamycin, chloramphenicol and rifampin were effective against more than 80% of B. fragilis isolates, At usual therapeutic doses, penicillin (PEN), ampicillin (AMP), cyclacillin (CYC), cephalothin (CT), cefazolin (CZ), cephradine (CPD), cefamandole (CMD), cefaclor (CCL), cefuroxime (CRX), spectinomycin (SPT), tetracycline (TET), ethambutol (EMB) and isoniazid (INH) were ineffective against B. fragilis. However, high and clinically tolerated doses of PEN, CMD and SPT inhibited more than 80% of B. fragilis isolates, but AMP, CYC, CT, CZ, CPD, CCL and CRX, TET, EMB and INH remained totally ineffective against B. fragilis. The therapeutic merits of these antibiotics are discussed.