Activity of purine nucleoside phosphorylase (EC 2.4.2.1) was estimated in thrombocytes of donors and of patients with various hematological diseases. The enzymatic activity was decreased in chronic lymphoid leukosis chronic myeloleukemia, chronic myeloleukemia and blast transformation, acute lymphoblast and myeloblast leukemia, myeloma. Dissimilar level of purine nucleoside phosphorylase activity was found in non-Hodgkin disease. The activity was only slightly increased in Waldenström's macroglobulinemia; it was similar to normal level in hypoplastic and autoimmune hemolytic anemias. A decrease in the enzymatic activity was observed in thrombocytes of the patients with hereditary forms of hemorrhagic diatheses (hemophilia A, Willebrand disease). Normal level of the enzymatic activity was noted in Glanzmann thrombasthenia.