Genetically anemic SI/SI(d) mice have been shown previously to have a defective hematopoietic environment which prevents extensive erythroid differentiation of normal hematopoietic stem cells and also confers resistance to the erythroleukemia-inducing virus, Friend spleen focus-forming virus (SFFV). In this study, we show that the relative resistance of SI/SI(d) mice to transformation by SFFV is not due to the inability of SFFV to replicate, nor is it because SFFV cannot transform erythroid cells, in the spleens of these mice. Injection of syngeneic +/+ mouse spleen cells, previously infected in vivo with SFFV, into secondary SI/SI(d) recipients resulted in marked splenic enlargement, and the appearance of large numbers of erythropoietin (Epo)-independent erythroid colonies in plasma clot culture. The cellular proliferation observed in these SI/SI(d) secondary recipients appeared to be due to infection and transformation of host SI/SI(d) cells rather than the growth of possible tumor colony-forming units (TCFU) present in the infected +/+ spleens, because preirradiation of the SI/SI(d) recipients abolished the splenomegaly and appearance of Epo- independent erythroid colonies. Furthermore, prior irradiation (1,200 rads) of donor spleen cells from SFFV-infected +/+ mice only slightly reduced spleen focus formation in unirradiated SI/SI(d) recipients. The conclusion that SI/SI(d) target cells could be infected and transformed by SFFV was confirmed directly by injecting a high titered preparation of SFFV into SI/SI(d) mice. SI/SI(d) mice were not absolutely resistant to infection or transformation by SFFV. Nevertheless, cells from the spleens of SFFV-infected mice were unable to form tumor colonies (TCFU) in irradiated SI/SI(d) recipients, suggesting that TCFU are either present at an undetectably low frequency in these spleens, or that they are still subject to the regulatory influences of the Steel locus.