The level of thymidine kinase activity in the premature leukocytes of patients with chronic myeloleukemia during the stable phase was shown to serve as a measure of the disease development. Considerable variations in thymidine kinase activity in blast cells in myeloid and lymphoid blast crises demonstrated that analysis of the enzyme activity might be used in the biochemical diagnosis of blast crisis in chronic myeloleukemia simultaneously with the enzymes of purine metabolism--ADA and PNP. During cell differentiation, the activity of thymidine kinase was decreased and in the myeloid cells the enzymatic activity was much higher of that in lymphoid cells as shown by investigations using blast cells of patients with blast crisis in chronic myeloleukemia, cells K-562, thymocytes, spleen and peripheric blood lymphocytes. Isozyme thymidine kinase I was mainly responsible for the rate of enzymatic activity in premature leukocytes of patients with chronic myeloleukemia regardless of the disease stage.