Minimal inhibitory concentrations of moxifloxacin were compared with those of ofloxacin, ciprofloxacin, clindamycin, metronidazole and six beta-lactams for 159 anaerobes isolated from human clinical samples. Unlike other fluoroquinolones, moxifloxacin demonstrated high activity against the 76 strains of the Bacteroides fragilis group as the minimal inhibitory concentration(50) was 0.5 mg/l. Porphyromonas, Prevotella, Fusobacterium and Gram-positive anaerobic cocci were inhibited by 1 mg/l or less of moxifloxacin. It inhibited 93.7, 94.9 and 98% of the 159 strains investigated at concentrations of 1, 2 and 4 mg/l, respectively. Moxifloxacin was more potent than ofloxacin and ciprofloxacin against Gram-positive rods and anaerobic cocci. Its broad anaerobic spectrum in vitro is promising for the treatment of intra-abdominal and respiratory infections.