A single exposure of mice of a low-leukemic strain (C57B1/6) to 400 r of total-body x-irradiation caused the early appearance of a factor which, though not itself capable of inducing leukemia in adult C57B1 recipients, did cause its appearance when the recipients also received urethan by injection. Positive results were obtained with seven different tissues from irradiated animals, whether urethan was injected 1 day or 30 days after the irradiation. No such effect was obtained in either series in the absence of urethan treatment. When urethan was injected into normal C57B1 mice, or into mice previously implanted with tissues from nonirradiated mice, only marginal leukemogenic activity was evinced. The possible involvement of a precursor virus is discussed.