In summary, LAP is an intriguing enzyme and its control is related to pituitary-adrenal function. A review of the changes in LAP activity which occur in some physiological conditions and in disease states has been presented. The function of LAP, however, is unknown. Table I summarizes those conditions in which the LAP is consistently altered enough so to help in the diagnosis of the disorder. Of prime importance is the differentiation of CML from a leukemoid reaction or agnogenic myeloid metaplasia with a leukocytosis. However, in no instance is the LAP value alone diagnostic of any disease. It remains a laboratory test to be utilized in conjunction with all other available clinical data.