The kinetics of co-administered ampicillin plus sulbactam were examined at doses being used in clinical trials of this combination of a beta-lactam antibiotic (ampicillin) and a beta-lactamase inhibitor (sulbactam). Following 15-min infusion of 2 g ampicillin plus 1 g sulbactam, peak serum concentrations of 120 micrograms/ml ampicillin and 60 micrograms/ml sulbactam were observed. The half-life was approximately one hour for both drugs. Approximately 75% of the dose of sulbactam and ampicillin was recovered in urine. The data fit a two-compartment pharmacokinetic model. Intramuscular administration of 1 g ampicillin plus 0.5 g sulbactam produced mean peak concentrations of 18 micrograms/ml ampicillin plus 13 micrograms/ml sulbactam. Estimates of variability of the data in normal male subjects are provided to serve as references for clinical trials.