The susceptibilities of 82 strains of the Bacteroides fragilis group to eight beta-lactam compounds, lincomycin, and metronidazole were determined by using an agar dilution technique. MK0787 (N-formimidoyl thienamycin) was the most active compound, inhibiting all strains at a concentration of 1 microgram/ml. Metronidazole was the only other drug of similar activity. Of the beta-lactam compounds, cefoxitin and MK0787 showed uniform activity against all species, whereas most other compounds were relatively less active against Bacteroides distasonis and Bacteroides thetaiotaomicron than against B. fragilis and Bacteroides vulgatus. Using a well diffusion technique, we determined the relative stability of each beta-lactam compound to sonicated cultures of selected resistant strains. Whereas MK0787 was completely stable to inactivation--and with one exception, cefoxitin was also--ceftriaxone, cefotaxime, cephaloridine, and cefoperazone always showed some inactivation, often quite substantial. Moxalactam and ceftazidime were completely stable to some of the enzyme preparations.