Piperacillin/tazobactam is a new drug consisting of a highly active penicillin and a beta-lactamase inhibitor. Pharmacokinetic variables of both components after they have been given together have been studied in healthy volunteers and in patients. Drug analysis in all studies was done by specific high pressure liquid chromatography (HPLC). In this review we summarise the pharmacokinetic properties of piperacillin/tazobactam. Most importantly, data on the piperacillin show that its behaviour is not changed when it is given with tazobactam. The pharmacokinetics of tazobactam are typical of beta-lactams, including the tissue penetration. It is distributed mainly into the extra-cellular space. Tissue concentrations of piperacillin/tazobactam and the concentrations of the two agents and their time course in plasma and tissue indicate that this combination is well formulated and truly synergistic pharmacokinetically. Clinical trials will show how these characteristics influence treatment outcomes.