Although previous studies have indicated that clavulanate may induce AmpC expression in isolates of Pseudomonas aeruginosa, the impact of this inducer activity on the antibacterial activity of ticarcillin at clinically relevant concentrations has not been investigated. Therefore, a study was designed to determine if the inducer activity of clavulanate was associated with in vitro antagonism of ticarcillin at pharmacokinetically relevant concentrations. By the disk approximation methodology, clavulanate induction of AmpC expression was observed with 8 of 10 clinical isolates of P. aeruginosa. Quantitative studies demonstrated a significant induction of AmpC when clavulanate-inducible strains were exposed to the peak concentrations of clavulanate achieved in human serum with the 3.2- and 3.1-g doses of ticarcillin-clavulanate. In studies with three clavulanate-inducible strains in an in vitro pharmacodynamic model, antagonism of the bactericidal effect of ticarcillin was observed in some tests with regimens simulating a 3.1-g dose of ticarcillin-clavulanate and in all tests with regimens simulating a 3.2-g dose of ticarcillin-clavulanate. No antagonism was observed in studies with two clavulanate-noninducible strains. In contrast to clavulanate. No antagonism was observed in studies with two clavulanate-noninducible strains. In contrast to clavulanate, tazobactam failed to induce AmpC expression in any strains, and the pharmacodynamics of piperacillin-tazobactam were somewhat enhanced over those of piperacillin alone against all strains studied. Overall, the data collected from the pharmacodynamic model suggested that induction per se was not always associated with reduced killing but that a certain minimal level of induction by clavulanate was required before antagonism of the antibacterial activity of its companion drug occurred. Nevertheless, since clinically relevant concentrations of clavulanate can antagonize the bactericidal activity of ticarcillin, the combination of ticarcillin-clavulanate should be avoided when selecting an antipseudomonal beta-lactam for the treatment of P. aeruginosa infections, particularly in immunocompromised patients. For piperacillin-tazobactam, induction is not an issue in the context of treating this pathogen.