Against 482 obligate anaerobes studied by the agar dilution technique, ceftizoxime was significantly more active than both cefoxitin and cefoperazone (P less than 0.001); the latter two agents were comparable in activity. The enhanced activity of ceftizoxime, as compared with the activity of cefoxitin, was against both gram-positive and gram-negative anaerobes (especially Lactobacillus and bacteroides spp). Cefoperazone, however, was more active than cefoxitin against gram-positive anaerobes (particularly Lactobacillus spp.) but was less active than cefoxitin against gram-negative anaerobes (particularly Bacteroides fragilis and Veillonella spp.).