The in vitro activity of BAY 12-8039 against 360 anaerobic clinical isolates was determined by the agar dilution method and compared to that of five other antimicrobial agents. BAY 12-8039 and imipenem were the most active agents tested. The following MIC90 values were determined for BAY 12-8039: Peptostreptococcus spp. (50 isolates), 1 mg/l; Propionibacterium acnes (30 isolates). 0.25 mg/l; Clostridium perfringens (30 isolates), 0.5 mg/l; Clostridium difficile (50 isolates), 2 mg/l; Bacteroides fragilis (50 isolates), 1 mg/l; non-fragilis Bacteroides, Porphyromonas, and Prevotella spp. (100 isolates), 2 mg/l; and Fusobacterium spp. (50 isolates), 0.25 mg/l. The results of the present study show that BAY 12-8039 may be useful in the treatment and prophylaxis of anaerobic infections.