Incidence of leukemia in RF mice exposed to a single whole body irradiation of 350 r X-rays was 77.4%, 41 out of 53 animals. Of them, 20 developed the thymic and other 20 the non-thymic lymphocytic leukemia, while only one developed myelogenous leukemia. Incidence of leukemia by 20-methylcholanthrene painting was 88%, 22 out of 25 animals. Of them, 12 developed lymphocytic leukemia and other 10 myelogenous leukemia. Experiments of inoculating cell-free filtrate prepared from these X-ray and 20-methylcholanthrene-induced leukemic RF mice revealed the following facts. 1) Cell-free filtrates from the leukemic mice which were induced by a single whole body X-irradiation were inoculated to the newborn mice of the same strain less than 16 hours old. Two (16.7%) of the twelve incculated animals developed leukemia 357 th and 374 th days, respectively, after the inoculation. Inoculation of the cell-free filtrates from normal mice to the newborn mice of the same strain did not produce leukemia. 2) Attempts to inoculate cell-free filtrate from 20-methylcholanthrene-indeced leukemic RF mice to newborn mice of the same strain were successful. 3) Cell-free filtrate from myelogenous leukemic mouse caused lymphocytic leukemia in one of three inoculated mice. This is suggestive of the possibility of transformation of the type of leukemia during cell-free transmission. 4) Intracerebral inoculation of cell-free filtrate from the brain of a mouse with lymphocytic leukemia also produced lymphocytic leukemia in one of two inoculated mice. 5) Inoculation of cell-free filtrate from the leukemic mice developed mammary carcinoma in one of three inoculated female mice. 6) Serial cell-free passage in two generations of 20-methylcholanthrene-induced leukemic mouse was possible. One of two inoculated mice developed leukemia in the first transmission and one of eight inoculated mice in the second transmission. 7) Only two cases of spontaneous lymphocytic leukemia were observed in RF mice during the past four years and the cell-free transmission of one leukemic mouse was also successful. 8) Virus-like particles were demonstrated by electron microscope in the lymph nodes of 20-methylcholanthrene-induced leukemic mouse. These facts suggest that the X-ray and chemical carcinogens may act to induce leukemia or mammary carcinoma by the "activation" of a latent virus naturally resident in the RF mouse.