The study of isoenzymes is often of great interest in clinical biology however, as far as alkaline phosphatase is concerned, no simple method is available to evaluate the forms of bony, intestinal, hepatic or placental origin. In most cases, it is necessary to combine several technic: electrophoresis, denaturation by heat, urea or phenyl alanine. This already complex analytical problem is rendered still more difficult by the choice of reference isoenzyme to be used calibration of the technic. The interest of this determination, especially in the field of oncology and pediatrics, should however encourage the biologist to pursue this route.