An alkaline phosphatase isoenzyme that did not move from the origin in agarose gel electrophoresis was detected in serum from a 51-year-old woman with Hodgkin's disease. Inhibitor and heat-inactivation studies of the patient's serum alkaline phosphatase showed properties resembling those of both liver and bone isoenzymes. No immunoglobulin or high-molecular-mass complexes with the alkaline phosphatase isoenzyme were detected. The relative molecular mass (Mr) of the atypical alkaline phosphatase isoenzyme was 182 000, that of the liver alkaline phosphatase isoenzyme control 170 000. Treatment of both of these isoenzymes with neuraminidase gave a product with an Mr of 140 000. We propose that a post-translational modification increased the carbohydrate content of the liver alkaline phosphatase isoenzyme, thus changing the charge characteristics of the enzyme and decreasing its electrophoretic mobility. We believe this to be the first report of a post-translational modification in a heat-sensitive isoenzyme of alkaline phosphatase.