The authors describe an unusual variant of alkaline phosphatase (ALP) discovered in a patient with a 90-fold increase in serum ALP. The variant ALP was indistinguishable from the bone isoenzyme when subjected to chemical inhibition, heat inactivation, lectin precipitation, and routine electrophoresis. However, treatment with neuraminidase produced a marked decrease in the ALP variant's electrophoretic mobility and a reduction in its molecular weight. A bone marrow biopsy revealed metastatic infiltration of the bone marrow by adenocarcinoma of the prostate accompanied by a remarkable amount of new bone formation, suggesting a bone origin for the unusual isoenzyme. The authors believe this to be the first report of an ALP variant with these properties.