Using biochemical and electron microscopic histochemical techniques, we studied membrane-bound alkaline phosphatase activities of rat hepatocytes and portal triads. Activity in portal triads was localized to capillaries surrounding bile ducts (peribiliary plexus) and arterioles. Despite the reputation of alkaline phosphatase as a "biliary enzyme," activity was not observed in bile ducts. Livers were separated into hepatocyte and portal triad fractions with collagenase. Enzyme from hepatocytes migrated faster during electrophoresis and eluted later during anion-exchange chromatography than that from portal triads. Thus, hepatocyte enzyme is more negatively charged (and also possibly smaller) than portal triad enzyme. Twelve hours after bile duct obstruction, new activity appeared on lateral and sinusoidal membranes of hepatocytes; appearance of portal triads did not change with obstruction. Electrophoretic mobilities of the two forms were not altered by obstruction. We conclude that two distinct liver alkaline phosphatases exist, one in hepatocytes, the other in portal triad blood vessels.