The behavior of the marrow megakaryocytes of leukemias before and after chemotherapy was observed by bone marrow tissue culture technic. Steroid hormone was used in acute leukemia and myleran in chronic for antileukemic chemotherapy. 1. Before treatment, the megakaryocyte count was less than normal level in acute leukemia and on the contrary, greater in chronic. The megakaryocyte function decreased both in acute and chronic leukmias. 2. After treatment, the megakaryocyte count was found markedly increased in chronic leukemia and frequently above normal level in acute leukemia. The megakaryocyte function also increased towards normal level both in acute and chronic leukemias and the increase was more remarkable in acute leukemia than chronic. 3. At relapse of acute lymphocytic and monocytic leukemias, both the megakaryocyte count and function showed no remarkable changes. 4. On the relation to the growth pattern, in acute leukemia, both the megakaryocyte count and function showed the tendency of decrease when the growth pattern proceeded from the intermediate pattern to the normal. In chronic leukemia, megakaryocyte function increased moderately and the count decreased markedly when the pattern proceeded from the normal pattern to the hypoplastic. From the results obtained and on the consideration of the hemorrhagic diathesis, it is advisable that in acute leukemia, antileukemic treatment should be given to maintain the normal pattern in bone marrow tissue culture and in chronic leukemia, administration of antileukemic agents is to be quitted as soon as the hypolastic pattern is observed and after a careful observation is paid on the bone marrow tissue clture findings and clinical signs and symptoms, the administration should be started again when the culture returns to the normal pattern.