For the elucidation of leukemogenesis of mouse leukemia induced by X-ray, a study was carried out on various factors influencing the development of leukemia. Namely, an effort was made to see the relationship between such factors as the age of mice receiving X-irradiation, the dose and the method of irradiation, the strain and sex of mice on one hand and the rate of leukemia induction on the other. Also a comparative study was made of the types of mouse leukemia elicited by X-ray. The results of the study are stated as follows. 1. In mice of the RF strain given one whole body X-irradiation of 350 r, and in both C(57)BL and C(3)Hf mice given four who1e body irradiations of 160 r each at the interval of 4 days to the total dosage of 600 r, the rate of leukemia induction was 77.3%, 88%, and 20% in the respective groups. 2. It has been demonstrated that the rate of leukemia induction was highest in mice of the RF strain 30-40 days old, and this rate decreased along with advancement in the age (several months old) of the mice. 3. As for the rate of leukemia induction in mice of the RF strain 30-40 days old, that received one whole body X-irradiation of 350 r, 22 (78.5%) out of 28 male mice developed leukemia while 19 (73.0%) out of 26 female mice, developed leukemia, showing no significant difference in sex. 4. It has been found that the rate of leukemia induction in the RF strain mice of 30-40 days old after one whole body X-irradiation increases with the dose of the irradiation, reaching its maximum at the dose of 350 r, but with the dose of 450 r the rate of leukemia induction rather decreases. 5. Of lymphocytic leukemia induced in the RF strain mice by X-irradiation, there were two types, that is, thymoma type and non-thymoma type, and there could be observed distinct differences in the peripheral blood pictures as well as in the patterns of cell infiltration into the liver between these two types. In addition, some of these lymphocytic leukemic mice presented a neutrophilic leukemoid reaction. There was only one mouse that developed myelogenous leukemia.