Monospecific antiserum to human placental alkaline phosphatase was purified by immunoabsorption and labeled with horseradish peroxidase. The binding of this labeled antibody to membrane fragments prepared from placental and tumor tissue was measured using agarose gel filtration to separate bound antibody. The antibody bound only to membrane fragments which contained placental phosphatase, and the amount bound varied in the order ascitic fluid membrane fragments > tumor extracts > placental extracts. Absorption of the antibody with crude placental membranes yielded a population of antibody which reacted with tumor tissue and pure placental enzyme, but only slightly with placental membranes. These results are interpreted to suggest that some antigenic sites are exposed in tumor tissue membranes which are not in placental membranes.

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This work was supported by Grant CA 23533-01 from the National Cancer Institute, NIH, and in part by the Gaston County Cancer Society.

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