Activities of the enzymes, responsible for degradation of purine nucleotides in leukocytes, were distinctly dissimilar in M1, M2, M4 and M6 variants of acute non-lymphoblastic leukosis studied in 34 patients. Differentiation of leukemic cells was shown to be due to alterations in activity of adenosine deaminase and purine nucleoside phosphorylase, which were oppositely directed as compared with those observed in lymphoblasts under conditions of acute lymphoblast leukosis. Evaluation of activities of adenosine deaminase, purine nucleoside phosphorylase and 5'-nucleotidase is of importance for characteristics of individual variants of acute non-lymphoblastic leukosis and for elucidation of the state of leukemic clone differentiation, which may affect the efficiency of the therapeutic measures used.