The interaction of the beta-lactamase inhibitors clavulanic acid, sulbactam and tazobactam with various plasmid and chromosomally mediated beta-lactamases from clinical isolates was studied. In each case inhibition of enzyme activity was observed to be progressive, i.e. the balance of the equilibrium between active and inactivated enzyme required 5 min. As expected, the clavam compound exhibited high affinity to all penicillinases including the enzyme from Staphylococcus aureus, but the affinity was poor to cephalosporinases. The affinity of tazobactam-a new penam sulfone-was 20- to 100-fold higher than that of sulbactam; its KI values ranged from 1.1 x 10(-9) to 5.0 x 10(-8) mol/l for class III and V enzymes (Richmond's and Sykes' classification) and from 4.1 x 10(-6) to 3.5 x 10(-5) mol/l for class I enzymes. Moreover, the affinity of tazobactam to the Enterobacter cloacae 2240 enzyme did not decrease at lowered pH values, but increased at alkaline pH values probably due to its binding to SH groups in the active centre of the enzyme.