The adenase activity was determined in erythrocytes, leucocytes and thrombocytes of donors and of patients with chronic lympholeucosis, chronic myeloleucosis and acute leucosis. A decrease in specific activity of the enzyme was observed in erythrocytes of the patients. In the leucocytes the adenase activity was not found both in normal persons and in all the patients examined. It was shown that the adenase is present in thrombocytes of the patients. Complete absence of the adenase activity was noted in the thrombocytes of healthy donors.