Thirteen antimicrobial agents and beta-lactamase inhibitor combinations were tested simultaneously for their in-vitro activity against a range of anaerobic Gram-negative bacilli with a standard reference agar dilution method. Overall, metronidazole, imipenem, ampicillin/sulbactam, ticarcillin/clavulanic acid and cefoperazone/sulbactam, followed by clindamycin, cefoxitin, and piperacillin, had the greatest activity. Cefotetan, ceftizoxime, and cefoperazone were moderately active, while ampicillin and penicillin were least active. Metronidazole was the only drug active against all strains, but only one strain was resistant to imipenem. Resistance was highest among certain members of the Bacteroides fragilis group, but was observed also among numerous other Bacteroides species. beta-Lactamase was produced by 94% of strains in the B. fragilis group, and by 64% of strains overall. The activities of clindamycin and cefoxitin were compared with those in previous surveys since 1982 at our institution. No clear evidence of increasing resistance was demonstrated, but the data emphasized the significant effects resulting from variations in susceptibility testing.