One hundred nine recent clinical isolates of anaerobic gram-negative bacteria were tested in triplicate by the National Committee for Clinical Laboratory Standards agar dilution procedure for their susceptibility to 32 antimicrobial agents. All isolates were inhibited by imipenem, but there were significant numbers of strains resistant to other beta-lactam drugs, and therefore the in vitro response to these antimicrobial agents cannot be predicted. This was particularly true for the bile-resistant or Bacteroides fragilis group. beta-Lactamase production was detected in 82% of the bacteroides with the nitrocefin test. Clavulanic acid combined with amoxicillin and ticarcillin and sulbactam combined with ampicillin resulted in synergistic activity against all beta-lactamase-positive organisms. Ceftizoxime was the most active of the cephalosporins. Two percent of the isolates were resistant to chloramphenicol and metronidazole. Clindamycin resistance was detected in 38% of the B. fragilis group, which is a marked increase from the 4% detected 10 years ago at this institution.