The agar dilution MIC method was used to test the activity of cefminox, a -lactamase-stable cephamycin, compared with those of cefoxitin, cefotetan, moxalactam, ceftizoxime, cefotiam, cefamandole, cefoperazone, clindamycin, and metronidazole against 357 anaerobes. Overall, cefminox was the most active -lactam, with an MIC at which 50% of isolates are inhibited (MIC₅₀) of 1.0 µg/ml and an MIC₉₀ of 16.0 µg/ml. Other -lactams were less active, with respective MIC₅₀s and MIC₉₀s of 2.0 and 64.0 µg/ml for cefoxitin, 2.0 and 128.0 µg/ml for cefotetan, 2.0 and 64.0 µg/ml for moxalactam, 4.0 and >128.0 µg/ml for ceftizoxime, 16.0 and >128.0 µg/ml for cefotiam, 8.0 and >128.0 µg/ml for cefamandole, and 4.0 and 128.0 µg/ml for cefoperazone. The clindamycin MIC₅₀ and MIC₉₀ were 0.5 and 8.0 µg/ml, respectively, and the metronidazole MIC₅₀ and MIC₉₀ were 1.0 and 4.0 µg/ml, respectively. Cefminox was especially active against Bacteroides fragilis (MIC₉₀, 2.0 µg/ml), Bacteroides thetaiotaomicron (MIC₉₀, 4.0 µg/ml), fusobacteria (MIC₉₀, 1.0 µg/ml), peptostreptococci (MIC₉₀, 2.0 µg/ml), and clostridia, including Clostridium difficile (MIC₉₀, 2.0 µg/ml). Time-kill studies performed with six representative anaerobic species revealed that at the MIC all compounds except ceftizoxime were bactericidal (99.9% killing) against all strains after 48 h. At 24 h, only cefminox and cefoxitin at 4× the MIC and cefoperazone at 8× the MIC were bactericidal against all strains. After 12 h, at the MIC all compounds except moxalactam, ceftizoxime, cefotiam, cefamandole, clindamycin, and metronidazole gave 90% killing of all strains. After 3 h, cefminox at 2× the MIC produced the most rapid effect, with 90% killing of all strains.