A useful laboratory test for the differentiation of liver, bone, and intestinal alkaline phosphatase (ALP) isoenzymes in serum is presented. Electrophoresis in polyacrylamide gel is performed with untreated serum as well as with serum incubated at 56 degrees C for 10 min. The heating step denatures bone isoenzyme which may obscure the liver ALP band when present in large amounts. Visualization of ALP activity is accomplished by the use of buffered p-toluidinium 5-bromo-4-chloro-indolyl phosphate and magnesium ions. In serum of patients with cholestatic liver disease, the occurrence of large molecular weight liver cell membrane fragments which contain ALP activity is postulated. These ALP-containing fragments occur at the origin of the electrophoretogram, unable to penetrate the small pore separation gel. Abnormalities involving ALP isoenzymes, such as bone isoenzyme arising from increased osteoblastic activity, may be detected. Intestinal isoenzyme, normally present in small amounts in some subjects of blood groups B or O, may be elevated in certain liver diseases, such as cirrhosis. By the use of this method the routine question of whether an ALP found to be increased in a screening procedure is due to liver or bone abnormality may be answered. In addition, the occurrence of abnormal ALP bands arising from cholestatic conditions and the occurrence of abnormal amounts of intestinal isoenzyme may also be detected.