One hundred and fifty strains of anaerobic bacteria including 45 bacteroides, 19 fusobacteria, 41 cocci, 34 clostridia, and 11 Gram-positive non-sporeforming rods were tested by agar dilution for their susceptibilities to cefoxitin, cefuroxime, latamoxef (moxalactam), penicillin G, chloramphenicol, clindamycin, metronidazole and Sch 34343. Excluding the 34 clostridia, 115 of the 116 remaining strains were inhibited by ≤ 1 mg/l of Sch 34343. One isolate of Bacteroides fragilis required 32 mg/l for inhibition. All of the 34 clostridia were inhibited by ≤ 8 mg/l of Sch 34343: 14 isolates of Clostridium difficile had an MIC50 of 4 mg/l, whereas the remaining 20 species of clostridia had an MIC50 of 0.125 mg/l and an MIC90 of 2 mg/l. On a weight basis, Sch 34343 was generally more active than any of the seven other antimicrobial agents tested.