Estimation of enzymes participating in degradation of purine nucleotides--adenosine deaminase (ADA), purine nucleoside phosphorylase (PNP), ecto-5'-nucleotidase as well as the ratio of ADA/PNP in leukocytes was shown to be of importance in differential diagnosis of acute lymphoblast leukosis subforms and for identification of a nature of malignized leukocytes clone in acute undifferentiated leukosis. Importance of these analyses is determined by specific differences in distribution of the enzymatic activity in mononuclear cells in T-, non-T-, non-B-cell acute lymphoblast leukosis and acute myeloblast leukosis as well as due to similar level and ratios of these enzymatic activities in most cases of acute undifferentiated leukosis and in acute lymphoblast leukosis.