One hundred clinical isolates of the Bacteroides fragilis group of bacteria were tested by agar dilution for susceptibility to cefazolin alone or in combination with clavulanic acid or sulbactam. For cefazolin, the MIC for 50% of the isolates (MIC50) was 32 micrograms/ml, the breakpoint for susceptibility. With the addition of 0.5 micrograms of clavulanic acid per ml, the MIC for 90% of the isolates (MIC90) was 8 micrograms/ml, well within the achievable range of concentrations in serum or tissue. Similarly, with the addition of 0.5 micrograms of sulbactam per ml, the MIC90 was 16 micrograms/ml. The addition of a higher concentration (4.0 micrograms/ml) of clavulanic acid or sulbactam resulted in MIC90S which were fourfold lower than those with 0.5 micrograms/ml. A fixed ratio of cefazolin-beta-lactamase inhibitor of 4:1 resulted in an MIC50 and MIC90 which were intermediate between the 0.5- and 4.0-micrograms/ml fixed concentration of beta-lactamase inhibitor.