1. Evidence of a marked difference in alkaline phosphatase activity in the leukocytes of normal subjects and individuals with chronic myelogenous leukemia has been provided by observations on the behavior of the enzyme following prolonged incubation and during pyogenic infection.

2. Therapy with radiation and radiomimetic drugs in chronic myelogenous leukemia and myeloid metaplasia resulted in a marked fall in leukocyte count but no change was observed in the relative population of alkaline phosphatase positive and negative cells.

3. In our laboratory histochemical and biochemical values for leukocyte alkaline phosphatase have been similar to those reported in the literature for typical cases of chronic myelogenous leukemia and most leukemoid reactions. However, in certain cases parodoxical findings have been noted in which histochemical and biochemical studies were inconsistent with the pathologic diagnosis.

4. The inadequacies of purely morphologic criteria in these atypical cases were noted and the desirability of the further development of cytochemical methods has been pointed out.

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