Several new beta-lactam antimicrobial agents have been introduced in the last few years. In this investigation, the in vitro activities of several recently introduced cephalosporins (cefoperazone, cefotaxime, ceftazidime, and ceftizoxime), moxalactam, and N-formimidoyl thienamycin were compared with those of cefoxitin, clindamycin, and metronidazole against 203 strains of anaerobic bacteria. At achievable serum levels, all of the antimicrobial agents were active against essentially 100% of the strains of anaerobic gram-positive cocci, Clostridium perfringens, Leptotrichia buccalis, and species of Selenomonas, Veillonella, and Eubacterium. Clindamycin, metronidazole, and N-formimidoyl thienamycin were the most active agents against the Bacteroides fragilis group, inhibiting all strains at concentrations which can be achieved in serum. Of the remaining agents tested against the B. fragilis group, cefoxitin (which required 64 mug/ml to inhibit 90% of the strains) was the most active, followed by cefoperazone (128 mug/ml), cefotaxime (128 mug/ml), moxalactam (128 mug/ml), ceftizoxime (256 mug/ml), and ceftazidime (>256 mug/ml). Important differences in cephalosporin susceptibility among species of the B. fragilis group were observed. Metronidazole and N-formimidoyl thienamycin were the most active drugs against species of clostridia other than C. perfringens; the other antibiotics displayed poor activity, although this is partly due to inclusion of a relatively large number of strains of Clostridium difficile which were very resistant to several of the cephalosporins. Only metronidazole was active against all species of Fusobacterium. Clindamycin and N-formimidoyl thienamycin displayed excellent activity against gram-positive, non-spore-forming bacilli, requiring </=8 mug/ml to inhibit 100% of the strains. Ceftazidime, cefoperazone, and moxalactam were bactericidal for 25 strains of B. fragilis at concentrations equal or close to those required for inhibition. On the basis of its activity in vitro, N-formimidoyl thienamycin appears to be the most promising of the new beta-lactam antibiotics for the treatment of infections involving anaerobic bacteria.