In rat liver, alkaline phosphatase is shown to exist in several distinct molecular forms originating from different cell types. The alkaline phosphatases of isolated hepatocytes and non-parenchymal liver cells were characterized with respect to electrophoretic mobility, thermostability, and sensitivity to treatment with neuraminidase in order to define the cellular distribution of the different enzyme forms within the liver. The major form of liver alkaline phosphatase could be attributed to the hepatocytic enzyme, whereas the properties of the minor form were found to be identical with those of the non-parenchymal cell enzyme. In contrast to the hepatocytic enzyme, that of the non-parenchymal cells revealed heterogeneity after desialylation.