The antibacterial activity of meropenem and comparative agents against approximately 1,000 anaerobes was determined using the disk dilution methods recommended by the National Committee for Clinical Laboratory Standards (NCCLS). The organisms represented 27 species of six genera and included the most common pathogens. Meropenem and imipenem were the most active drugs and were comparable in overall activity, generally exhibiting an MIC90 of < or = 1 micrograms/mL. In contrast, the MICs of cefoxitin, clindamycin, and metronidazole were 32, 16, and 2 micrograms/mL, respectively. Meropenem was two- to fourfold more active than imipenem against selected Bacteroides species, Clostridium species, and Fusobacterium species. At a concentration of 1 microgram/mL, meropenem was more active than imipenem against cefoxitin-resistant Bacteroides fragilis or Bacteroides thetaiotaomicron. At a concentration of < or = 0.5 micrograms/mL, meropenem was more active than imipenem against clindamycin-resistant Bacteroides distasonis. At a concentration of 2 micrograms/mL, meropenem was more active than imipenem against cefoxitin-resistant or clindamycin-resistant Clostridium difficile. Thus, meropenem's high potency and broad-spectrum activity against common, rare, and drug-resistant anaerobes confirms its utility in the treatment of mixed anaerobic and aerobic infections.