Patients comparable in disease, therapy and serum bilirubin concentration were either treated with mezlocillin intravenously or not at all. The bile of each patient was collected either from a T-drainage or from a percutaneously placed drainage into the bile ducts. The concentrations of GGT and AP, which were liberated by destroyed liver cells, and of bilirubin and mezlocillin, which were secreted actively, were analysed. Those patients who had normal serum bilirubin concentrations had a significantly higher biliary bilirubin excretion than those with high serum bilirubin level. The maximum excretion was after 4 hours. While the biliary concentration of bilirubin decreased, the concentration of secreted mezlocillin increased. Due to destroyed liver cells those patients with pathologically elevated blood bilirubin levels had a 50-fold lower mezlocillin excretion than those with normal blood values.