In 80 patients with AML, ALL, BC-CML and other proliferative blood diseases, the sensitivity of blast cells to cytostatics (vincristine, cytosine arabinoside, daunorubicin, l-asparaginase, hydrocortisone) was determined. The studies were made with the use of liquid scintillation measuring the inhibition of 3HTdR incorporation induced by the cytostatics tested. It has been shown that the method makes it possible to demonstrate individual reactivity of acute leukemia blasts to cytostatics. Variations in the individual sensitivity to cytostatics were mostly seen in acute myeloblastic leukemia, blastic crisis of chronic myelocytic leukemia, less often in acute lymphocytic leukemia.