The number of transient endogenous spleen colonies (TEC) formed on days 4-6 postirradiation was found to be greatly increased in mice inoculated with M. arthritidis 1 day prior to sublethal irradiation. In irradiated plethoric mice, TEC formation was reduced (about 10-fold), but preinfection with M. arthritidis reversed the effect of plethora, and TEC formation in infected plethoric mice was almost as high as in infected normal mice. An abortive wave of 59Fe uptake was revealed as early as 5-6 days postirradiation in the spleen of control and infected mice. At day 6 a similar transient peak of erythropoiesis was observed in the marrow of mice infected with M. arthritidis. The data attest to the possible existence of the erythropoietin-independent mechanisms of the mycoplasma-induced activation of transient endogenous colony-forming units. The latter may play a major role in the development of various impairments of hematopoiesis and immune reactivity, which were often observed in mice infected either with mycoplasma or dually infected with mycoplasma and virus species.