Activity of ornithine decarboxylase was studied in human leukocytes exhibiting various proliferative activity. The enzymatic activity might be estimated without obligatory destruction of the cells and fractionation of the cell lysates. In leukocytes of healthy persons the rate of ornithine decarboxylase activity depended on belonging of the cells to myeloid or lymphoid forms: the enzyme activity was higher in lymphocytes as compared with granulocytes. In blast cells of blood of patients with acute leukosis the higher activity of ornithine decarboxylase was found as compared with adult leukocytes although it did not exceed the enzymatic activity in medullar cells of healthy persons. Evaluation of the dynamic alterations in ornithine decarboxylase activity in leukocytes of the patients with leukoses might be used as a test for early determination of the actively proliferated hemopoietic cells.