The alkaline phosphatase isozyme pattern of blood plasma and liver was studied in rats which were subjected to various treatments such as tumor inoculation, liver injuries and the ligation of bile duct, by means of agar-gel electrophoresis, gel-filtration on Sephadex G-200 and anion-exchange column chromatography on DEAE-Sephadex A-25. Acid phosphatase activity was also determined in some of these animals. The following results were obtained. (1) The elevated level of plasma alkaline phosphatase was shown in rats whose livers were experimentally damaged. This elevation was most remarkable when hepatobiliary system was occluded. (2) The alkaline phosphatase activities of plasma and liver, and plasma icterus index were found to increase in parallel in rats with liver metastases of tumor or with their bile ducts ligated, but not in those with the liver injured by CCl₄ treatment. (3) By agar-gel electrophoresis, four kinds of isozyme pattern of this enzyme were observed in this experiment. (4) After gel-filtration on Sephadex G-200, rat serum proteins were separated into three peaks and alkaline phosphatase activity was found only in the second peak. The enzyme was further separable into two peaks when the second peak eluent from the rat with damaged liver was chromatographed on DEAE-Sephadex A-25. (5) In rats having ligated bile duct, the acid phosphatase activity of plasma showed no noteworthy changes throughout the entire period.