Transplantation of the liver in man is frequently complicated by biliary fistula and obstruction of the biliary tree by casts, which suggests that the composition of the bile may be abnormal. In part of the present study, bile composition was investigated in three recipients during the first few weeks after transplantation, when a T tube was in place. Supersaturation of bile with cholesterol was found in two patients immediately after surgery and during episodes of acute rejection, but bile was never lithogenic in the third. There was no evidence of bile stasis in the absence of acute rejection and bile viscosity was normal in all three patients throughout the study. Free bile acids and free bilirubin, which are relatively insoluble products of bacterial metabolism, were not present in the bile of any patient. However, chemical analysis of casts found at autopsy in a fourth recipient showed that the major component was free bilirubin. Escherichia coli was grown in cultures of the casts and the organisms were shown to possess glucuronidase activity, thus providing an explanation for the high bilirubin content. There was also some evidence in one case that damage to the bile duct mucosa has led to the precipitation of material upon it, and it is concluded that a number of factors, including infection, supersaturation with cholesterol, and mucosal damage, may be involved in bile case formation after transplantation of the liver.