Acrylamide gel disc electrophoresis provides a reliable and reasonably rapid method of differentiating the raised serum alkaline phosphatase (AP) of bone origin from that of liver origin. The technique has been placed for the first time on a semiquantitative basis. Measurement of both band width and band position effectively distinguishes the bone from the liver isoenzyme, but band width provides superior discrimination. An origin band was seen in none of the normal subjects and in only 7% of patients with bone disease but was present in 78% of patients with liver disease, a highly significant increase. Fifty percent of normal individuals had a small-intestinal band in serum taken two hours after a meal, as did 35% of patients with liver disease, but the incidence of intestinal bands in bone disease was only 11%, significantly less than in the other two groups. The genetic control of small-intestinal AP in serum has been confirmed, but it has been demonstrated that the decrease of intestinal AP in bone disorders is not genetically determined.