The agar dilution technique was used for determination of the bacteriostatic activity of carbenicillin, penicillin G, cefazolin, cephaloridine, cefoxitin, and cephalothin agaomst a variety of anaerobic bacteria. Carbenicillin showed a high level of activity at a concentration of smaller than or equal to 100 mug/ml; only five of 123 strains of Bacteroides fragilis, one strain of Bifidobacterium eriksonii, and one strain of Clostridium bifermentans were resistant to a concentration of larger than or equal to 100 mug/ml. Cefoxitin, a beta-lactamase-resistant drug, was highly active against B. fragilis and most other anaerobes at a concentration of smaller than or equal to 32 mug/ml; the exceptions were one strain of Bacteroides species and 13 of 28 strains of Clostridium species. The other cephalosporins were less active against B. fragilis but exhibited good activity against most of the other strains tested. Bactericidal concentrations of cefoxitin and cephalothin were determined for 51 selected strains by the broth dilution technique, and the activities of these drugs were compared with those of two other drugs (clindamycin and metronidazole) known to be very active against anaerobes. Metronidazole wasthe most consistently bactericidal of the four drugs tested for this activity.