In order to clarify the leukemogenesis by x-ray, observations were carried out on the peripheral blood pictures and on the histological changes of hematopoietic organs from time of X-irradiation to the onset of leukemia with lapse of time, and in addition, the developmental course of leukemia was pursued by Hiraki's clinical tissue cultures. As the result the following conclusions were drawn. 1. After one whole body irradiation of 350r on the RF strain of mice, leukemia developed in 77.3% of them. Out of them 75.4% proved to be lymphocytic leukemia while 1.8% myelogenous leukemia. In the former group some were of the thymoma type that presented swelling of the thymus and the others of the non-thymoma type without such swelling of the thymus. 2. After the X-irradiation, these mice showed hypoplasia of hematopoietic organs, which was followed by reactive proliferation, and in those of the thymoma type leukemic changes originated in the thymus whereas in those of the non-thymoma type the changes commenced in the spleen or in lymph node. 3. One month before the onset of leukemia, that is, in the so-called preleukemic stage, there could be observed a decrease in the leukocyte counts and an increase in the eosinophil counts, and in addition, the splean, lymph nodes or thymus showed such findings what might be histologically considered as leukemia, but by the clinical tissue cultures the findings rather suggested that it was in the stage showing hypoplasia. From these findings it has been concluded that leukemia induced by X-ray irradiation originates in the thymus in the case of lymphocytic leukemia of the thymoma type, while in the case of lympocytic leukemia of the non-thymoma type leukemia makes its appearance first in the spleen or in lymph nodes after passing a hypoplastic stage.