Investigation of mild, inherited increased serum alkaline phosphatase activity partially combined with Gilbert's syndrome in one family showed, apart from a normal liver fraction, an intestinal isoenzyme pattern and an extra band in the agar electrophoresis. Analysis by agarose electrophoresis before and after incubation of neuraminidase showed that the extra fraction was an intestinal variant isoenzyme. The precise genetic background of the two disorders in this family could not be determined from the available data. Abnormal activities of (regular) intestinal alkaline phosphatase isoenzyme caused the increase in serum alkaline phosphatase in the absence of disease.