Alkaline phosphatase (ALP) was purified from the rat kidney. The appropriate enzyme protein was injected subcutaneously into a rabbit as an antigen. The obtained antiserum showed a single precipitin line in the double diffusion test of Ouchterlony. The localization of ALP in the rat liver was examined by means of immunohistochemical techniques. At the light microscopic level, positive reactions were recognized along cell borders between adjacent hepatocytes. On examination with an electron microscope, gold particles were observed only on the surface of microvilli of bile canaliculi by immunogold staining. On the other hand, by the peroxidase-labeled antibody method, reaction products appeared not only on the bile canalicular membrane but also on the sinusoidal and lateral membranes.