As one of the series of studies on radiation leukemogenesis, changes in the peripheral blood picture occurring with lapse of time were pursued with the Cb, D103, StA, and Zb strains of mice (30-50 days old) after a single whole body X-irradiation of 350 r. The results are summarized as follows. 1. Both erythrocyte counts and hemoglobin contents showed the minimum values 2 weeks after the irradiation, and after recovering once they tended to decrease again. Reticulocytes decreased to about 2-4‰ immediately after the irradiation but by the second week they increased to about twice the number in normal condition, and thereafter they returned to the normal count. This change was most marked in the D103 strain, but ultimately the changes in the erythrocyte series presented similar curves in all the four strains mentioned above, and these changes did not differ from those observed in RF strain. 2. The number of leukocytes decressed rapidly to as low as 2,000 immediately after the X-irradiation. The susceptibility to X-ray irradiation was higher in the leukocyte series than in the erythrocyte series. 3. Lymphopoiesis was more sensitive to X-irradiation than myelogenous hematopoiesis, and the decrease of lymphocytes was greater than that of neutrophils, showing a relative neutrophilia for several weeks after the irradiation. 4. After once recovering, the leukocyte counts in the Cb, D103, StA, and Zb strains of mice all did not show such a transient decrease as observed in the preleukemic stage of RF strain about 13 weeks after the irradiation. In addition, lymphocytic leukemia did not develop. 5, In the Cb, D103, StA, and Zb strains irradiated with X-ray there could be recognized no severe eosinophilia such as observed in the RF strain that received X-irradiation. From these findings it has been confirmed that there can be recognized no marked differences in the peripheral blood picture immediately after a single whole body X-irradiation of 350 r between the RF strain of mice that develop leukemia by X-ray, and the Cb, D103, StA. and Zb strains that do not develop leukemia by X-irradiation, but in the observations conducted thereafter along with lapse of time there can be seen several distinct differences in the changes of the leukocyte series between them.