Piperacillin/tazobactam is a new combination of a broad-spectrum penicillin and a beta-lactamase inhibitor. In studies in healthy volunteers, the pharmacokinetics of piperacillin combined with tazobactam were similar to those of piperacillin alone. In contrast, tazobactam administered with piperacillin achieved higher plasma concentrations and had a longer half-life than tazobactam administered alone. Intravenous infusion of 4.0 g piperacillin with 0.5 g tazobactam over 5 min resulted in mean maximum plasma concentrations of 380 micrograms piperacillin/ml and 35.3 micrograms tazobactam/ml; half-lives were 1.14 h for piperacillin and 0.92 h for tazobactam. Within 30 min of infusion, piperacillin/tazobactam achieves 16-85% of plasma concentrations in skin, muscle, lung, gallbladder, and intestinal mucosa. Plasma and tissue levels remain above the MIC90s of major pathogens for 2 h post administration. These findings show that piperacillin/tazobactam is truly synergistic combination which can be expected to be effective in treating a wide variety of infections in the clinical setting.