1. A monoclonal antibody APP, 1 against harp seal alkaline phosphatase has been prepared. It was found that the antibody was cross-reacted with the intestinal alkaline phosphatase of common seal Phoca vitulina larga. 2. Purified antibody was linked to Sepharose 4B and used for immunoaffinity chromatographic purification of alkaline phosphatase from the intestinal content of common seal. A spec. act. of the purified enzyme was 7300 units per mg of protein. 3. The enzyme in 7.5% polyacrylamide gel in the presence of 2-mercaptoethanol and SDS was migrated as a single band of Mr 67,000. The value of the apparent Km for common seal alkaline phosphatase was equal to 3.7 mM.