AbstractWe have used enzyme specific inhibitors and heat inactivation to distinguish Leukocyte alkaline phosphate (LAP) from other organ‐specific alkaline phosphatases as well as to compare LAP from normal granulocytes and leukemic cells with elevated LAP. The heat inactivation and inhibition curves of LAP are quite different from those of other organ‐specific alkaline phosphatases. The inhibition curves and heat inactivation characteristics of LAP from normal granulocytes and that obtained from chronic granulocytic leukemia (CGL) blast phase cells with elevated LAP are identical. These data suggest that LAP is distinct from other organ‐specific alkaline phosphatases, particularly placental alkaline phosphatase. We also conclude that the LAP present in cells with elevated levels is very similar or identical to that of normal granulocytes.