Cefoperazone is a semi-synthetic cephalosporin for parenteral use with an extended antibacterial spectrum covering Pseudomonas aeruginosa, Enterobacter cloacae and Serratia marcescens. Its pharmacokinetic properties were studied in 8 healthy subjects after 2 intravenous infusions of 2 g of the drug at a 12-hour interval. The mean peak serum concentrations were 134 +/- 16 microgram/ml and 143 microgram/ml. Cefoperazone was shown to possess a long half-life for a cephalosporin (1.7 hours). In our concentration range the drug is 90% protein bound. The apparent volume of distribution was a mean 11.4 liters and the renal clearance 18 ml/min. The cumulative urinary excretion was small, viz. 23% in 12 hours, indicating that there should be no need to modify the dosage regimen in renal failure. Comparison of in vitro studies with the pharmacokinetic properties show that 2 g cefoperazone given intravenously twice a day should inhibit most sensitive bacteria.