Crystalline human alkaline phosphatase from placenta and intestine was isolated by butanol extraction, acetone precipitation, a heat step (for the placental enzyme), ammonium sulfate precipitation, anion exchange chromatography, gel filtration and crystallisation with ammonium sulfate. Rabbit antibodies showed a partial cross-reaction between both enzymes in double diffusion, quantitative precipitation experiments and in serial precipitin curves. There was no reaction with human alkaline phosphatases from liver, kidney and bone. Three phenotypes of placental alkaline phosphatase with different electrophoretical mobility exhibited identical immunological reactions with their respective antisera. Monospecific antisera were obtained by absorption with crystalline intestinal or placental alkaline phosphatase. These monospecific antisera against the placental or the intestinal alkaline phosphatase can be used for an immunological determination of these two alkaline phosphatases without contamination by other alkaline phosphatases in human serum.