11 patients with cholelithiasis and choledocholithiasis, respectively, were cholecystectomized with subsequent inspection of the common bile duct. The drainage of the bile alpha-amino-3,6-dihydrobenzylpenicillin (epicillin, Spectacillin) were administered i.v. and the antibiotic concentrations in the bile and the serum determined. Samples were taken at intervals of 20 min over a period of 3 h. The maximum concentration in the bile was observed 60 min following administration of the antibiotic. The values in the bile were found to exceed the corresponding serum concentrations 6--14 fold. By the end of the observation period the concentrations of epicillin in the bile as well as in the serum were still higher than the minimum inhibitory concentrations for most of the gram-positive and gram-negative organisms.