Piperacillin, a new injectable synthetic penicillin, was evaluated against biliary tract infections. 1. Two grams of piperacillin was intravenously administered to patients received cholecystectomy. The mean level in gallbladder bile of PIPC was 795.6 microgram/ml except for 3 cases in obstruction of the cystic duct. The mean gallbladder tissue level was 31.2 microgram/g. The gallbladder tissue level in the cases with obstruction of the cystic duct was high levels as 71.3 microgram/g and 79.5 microgram/g. 2. The excretion of PIPC into bile was compared with TIPC and APPC using crossover method. When administered 2 g of PIPC, the peak biliary levels were 950 microgram/ml to 2,120 microgram/ml at 2 hours and 20 minutes to 2 hours and 40 minutes after the administration, and biliary recoveries during 6 hours were 2.84% to 11.6%. The peak levels and biliary recoveries were lower after administration of TIPC 2 g. Mn and Zn were excreted enormously together with APPC into human bile. 3. The influence on clinical laboratory findings was negligible. Therefore, PIPC may be expected to show excellent effects of biliary tract infections except rare occurrence of drug eruption.