Fifteen strains of Bacteroides fragilis, five highly resistant, five moderately resistant, and five susceptible to benzylpenicillin and cephaloridine, were tested for beta-lactamase production. In the highly resistant and the moderately resistant groups of strains, a correlation between formation of beta-lactamase and minimal inhibitory concentrations was demonstrated. In the presence of the beta-lactamase inhibitors clavulanic acid and CP-45,899 at concentrations of 1 mug/ml, the susceptibility to cephaloridine increased fourfold or more in the beta-lactamase-producing strains. Crypticity measurements (beta-lactamase activity broken/intact cells) with cephaloridine as substrate could indicate a diffusion barrier in the cell wall.