The aim of the present investigation was to determine the in vitro activity of HMR 3647 compared with other antimicrobial agents against anaerobic bacteria. The activity of HMR 3647 was determined against 342 clinical isolates of anaerobic bacteria by the agar dilution method and was compared with azithromycin, clarithromycin, roxithromycin, erythromycin, cefoxitin, imipenem, clindamycin and metronidazole. Among the macrolides HMR 3647 and among the beta-lactams imipenem were the most active agents tested. Anaerobic cocci (50 strains) had the following minimum inhibitory concentrations (MICs): HMR 3647, range 0.016-0.125 mg/l; imipenem, range 0.016-0.064 mg/l. Propionibacterium acnes (30 strains): HMR 3647, 0.016-1.0 mg/l; imipenem, 0.032-0.064 mg/l. Clostridium perfringens (30 strains): HMR 3647, 0.125 mg/l; imipenem, 0.016-0.5 mg/l. Clostridium difficile (50 strains): HMR 3647, 0.125-256 mg/l; imipenem, 4.0-8.0 mg/l. Bacteroides fragilis (102 strains): HMR 3647, 0.032-16 mg/l; imipenem, 0.064-0.25 mg/l. Bacteroides and Prevotella species (50 strains): HMR 3647, 0.016-4.0 mg/l; imipenem, 0.016-0.25 mg/l. Fusobacterium nucleatum (30 strains): HMR 3647, 0.016-8.0 mg/l; imipenem, 0.008-0.064 mg/l. HMR 3647 may be useful as treatment and prophylaxis for infections due to anaerobic bacteria.