M. arthritidis is known to enhance the leukemogenic effect of Rauscher leukemia virus in mice of different strains. As we have found earlier, M. arthritidis enhanced endogenous colony formation in irradiated mice. In this paper we made an attempt to establish a connection between these two phenomena. The mixed inoculation of BALB/c mice with M. arthritidis and Rauscher leukemia virus produced a great increase in the number of endogenous colonies as compared with the effect of mycoplasmas alone, while Rauscher leukemia virus did not enhance the number of colonies. When injected into mice a day before their lethal irradiation, M. arthritidis was shown to increase the survival of mice within 30 days following irradiation. We tried to determine the possible mechanism of the effect of M. arthritidis on hemopoiesis. Our findings led us to the conclusion that the mycoplasmas had no influence on the splenic microenvironment and did not increase the migration of the surviving stem cells to the spleen. It is conceivable that M. arthritidis acts not on the pluripotent stem cells, but rather on the committed precursor cells. If so, the mechanism responsible for the enhancing effect of mycoplasmas on the development of Rauscher leukemia and endogenous colony formation resides possibly in the capacity of M. arthritidis to increase the number and/or sensitivity of target cells to the virus.