Piperacillin is a new semisynthetic, expanded-spectrum penicillin with marked activity against Pseudomonas aeruginosa. The biliary excretion of piperacillin was studied in patients undergoing cholecystectomy. Concentrations of piperacillin in common duct bile at 35 to 90 min postinfusion of 1-g doses ranged from 31 to 920 micrograms/ml, with a mean (+/- standard deviation) of 467 +/- 363 micrograms/ml. Gallbladder piperacillin levels at 30 to 75 min postinfusion ranged from 2.2 to 80 micrograms/ml, with a mean of 27 +/- 31 micrograms/ml. No correlation occurred with peak serum level of antibiotic, creatinine, bilirubin, or alkaline phosphatase. Significant amounts of piperacillin were excreted via the biliary system.