Agar dilution was used to compare the in vitro activity of CP 99,219 with those of ciprofloxacin, grepafloxacin, metronidazole, cefoxitin, piperacillin, and piperacillin-tazobactam against 489 anaerobes. CP 99,219 yielded a MIC for 50% of the strains tested (MIC50) of 0.25 micrograms/ml and a MIC90 of 1.0 microgram/ml, with 99.6% of the strains susceptible at a breakpoint of 2.0 micrograms/ml. Ciprofloxacin and grepafloxacin were less active (MIC50, 4.0 micrograms/ml; MIC90, 32.0 micrograms/ml and 2.0 and 16.0 micrograms/ml, respectively). Metronidazole was active against all gram-negative rods (MIC90, 4.0 micrograms/ml), but 31% of the gram-positive anaerobes were resistant at > 8.0 micrograms/ml. Cefoxitin was active against 84% of all strains at < or = 16.0 micrograms/ml, with a MIC50 of 4.0 micrograms/ml and a MIC90 of 32.0 micrograms/ml. Tazobactam enhanced the activity of piperacillin against > 95% of the beta-lactamase-producing gram-negative anaerobic rods (MIC90, 16.0 micrograms/ml).