The experiments here described were begun five years ago. In a personal communication Doctor E. L. Opie called our attention to the relatively frequent occurrence of leukemia among radiologists. This suggested that the incidence of leukemia in our colony of mice might be increased by irradiation with x-rays. Aubertin9s more recent communication (1) leaves little, if any, doubt as to the role of x-rays in leukemia in radiologists. Krebs, Rask-Nielsen and Wagner (2) were the first to study this problem. They irradiated 5550 mice with x-rays and found that the incidence of lymphomatosis among the x-rayed animals was 3.6 per thousand as compared with only 0.58 per thousand among unirradiated controls. No mention is made in their report as to the age and sex of the experimental animals or of diseases other than lymphomatosis. A preliminary analysis of our data (3) indicated that not only was the incidence of lymphomatosis enhanced by irradiation, but also that of myelosis, and that the greatest percentage increase among the different varieties of lymphomatosis was in mediastinal lymphosarcoma. Successful transmission of myeloid leukemia occurring among x-rayed mice has been reported in a previous communication (4). The causation of ovarian tumors by x-rays was clearly recognized by us about a year ago (5).