The in vitro activities of several new beta-lactam-beta-lactamase inhibitor combinations (piperacillin plus tazobactam, ceftizoxime and cefonicid with sulbactam and clavulanic acid, and ampicillin plus 8 micrograms of sulbactam per ml) were tested with anaerobic bacteria and compared with known beta-lactam-beta-lactamase inhibitor combinations and other potent antianaerobe agents. All the combinations tested (except for the cefonicid-inhibitor combinations) were active against almost all strains of the Bacteroides fragilis group. This report indicates that beta-lactamase inhibitors may improve the activity of beta-lactam agents with marginal activity against the B. fragilis group.