In experimental bile obstruction the serum activities of the membrane-bound liver enzymes, alkaline phosphatase, 5'-nucleotidase and gamma-glutamyltransferase are greatly increased, whereas in the liver only the alkaline phosphatase activity is elevated. After partial hepatectomy or tetrachloride poisoning the alkaline phosphatase activity in the regenerating live is increased to the same extent as in cholestasis without an accompanying elevation in serum activity. The following results support the hypothesis of a bile salt-mediated solubilization of membrane-bound enzymes in cholestatic liver: (1) 30 min after bile duct ligation the total bile acids in the liver were increased 5-fold, 2 h later as much as 10-fold. After 1 day, the bile acid concentration was still 4 times above normal. (2) Isolated plasma membranes from normal and obstructed livers were incubated in vitro with increasing amounts of tri- and dihydroxycholanic acids. At a final concentration of 1 mmol/l taurochenodeoxycholate significant amounts of membrane-bound enzymes were released into the 12,000-g supernatant. (3) In the regenerating liver, where tissue phsophatase activity was high and serum phosphatase activity unchanged, the bile salt concentration was not increased.