In cultured human lymphoblasts methionine synthase(MS)-activity is enhanced during the logarithmic phase of population growth. MS is a ‘logphase enzyme’. Little activity is found in normal leukocytes and in cells of patients with chronic myelocytic leukemia. Acute myelocytic and lymphocytic leukemias show markedly enhanced values. Compared to thymidine kinase and DNA-polymerase, the very stable MS gives the same possibility to control the treatment of leukemic patients.