Inactivation of a beta-lactamase of Streptomyces cacaoi by clavulanic acid and PS-5 was investigated and compared with that of a beta-lactamase of Bacillus cereus. Inhibition of the enzymes induced by clavulanic acid and the beta-lactam antibiotic PS-5 was found to be progressive with time. However, the degree of inhibition of the beta-lactamase from S. cacaoi increased more progressively with time than that of the enzyme from B. cereus. Conformative response constants were determined. As compared with clavulanic acid, over ten times higher concentrations of PS-5 were necessary to give a similar degree of inhibition. At lower concentrations, both clavulanic acid and PS-5 behaved as competitive inhibitors. Ki values calculated from the integrated form of the LINEWEAVER-BURK type were 1.1 X 10(-7) M and 7.6 X 10(-6) M for clavulanic acid and PS-5, respectively.