The pharmacokinetic properties of piperacillin/tazobactam are summarized. The data on piperacillin show that the behaviour of this well established agent is the same when given in combination with tazobactam as when given alone. Tazobactam shows pharmacokinetic characteristics typical of a beta-lactam compound. It distributes mainly into the extracellular space and shows typical tissue penetration behaviour. The tissue concentrations of piperacillin and tazobactam and their pharmacokinetic profiles in plasma and tissues encourage the view that the synergy observed in vitro will be reflected in clinical use.