1. The excretion of piperacillin sodium in bile was studied after intravenous injection of 2 g (n = 5) and 4 g (n = 7). All patients had undergone cholecystectomy and exploration of the common bile duct for cholelithiasis. Bile, serum and urine concentrations were measured by a microbiological technique. 2. Peak concentrations of piperacillin were found in bile at 150 minutes after 4 g and at 210 minutes after 2 g. 3. Both the concentration of piperacillin in bile at the end of a 6 1/2 hour study and the calculated mean bile concentration of piperacillin were above the minimum inhibitory concentration (MIC) of this drug for organisms commonly found in acute biliary tract infections. We believe piperacillin may be useful in the management of such conditions.