The biliary excretion of cephalothin has been studied both by perfusion of isolated rabbit livers and during biliary drainage in the human. In the perfused rabbit liver, cephalothin added to the circulating blood is partly eliminated in the bile where the antibiotic activity is similar to that of the serum. Following a single injection of 1g to 10 cholecystectomized patients, the maximal concentration in the bile collected by external drainage reached after 1–3 h an average of 15.5μg/ml. Simultaneous determinations showed higher levels of cephalothin concentration in the gallbladder bile than in the common duct bile. In two patients with impaired renal function, cephalothin concentration in bile obtained by duodenal drainage was found to be more elevated than in five normal subjects. Biliary elimination of cephalothin appears to be sufficient to justify the use of this antibiotic in the treatment of biliary tract infections.