The in vitro activities of a new quinolone, WIN 57273, and the combination of piperacillin and tazobactam, a beta-lactamase inhibitor, were compared with those of cefoxitin, ceftizoxime, chloramphenicol, clindamycin, imipenem, metronidazole, and piperacillin for 123 clinical anaerobic isolates. Ceftizoxime and cefoxitin had equivalent activities, while metronidazole was active against gram-negative isolates. In the Bacteroides fragilis group, species other than B. fragilis were the most resistant. The combination of piperacillin with tazobactam in a ratio of 8 to 1 was more effective than piperacillin against B. fragilis group organisms when the MIC of piperacillin was greater than or equal to 64 micrograms/ml. Overall, WIN 57273 (i) and imipenem (ii) were the most active agents, with MICs for 50 and 90% of strains of (i) 0.25 and 0.5 and (ii) 0.125 and 2 microgram/ml, respectively.