Summary Germfree rats of 3 strains (Wistar, Fischer and Sprague-Dawley) were examined for leukemogenic potential, (a) Particles of indefinite nature were observed only in 2 of 10 Wistar rats, (b) Leukemia could not be induced in the rats by whole-body administered X-rays, and (c) germfree Sprague-Dawley rats were as susceptible to Gross passage A leukemia virus as conventional rats.