Examined were the antigenic properties and content of type-C virus particles of a series of lymphomas induced by radiation leukemia virus (RadLV) in Wistar Furth (W/Fu) inbred rats. In conformity with previous studies, adult syngeneic rats ordinarily reject these tumors unless these recipients are irradiated. Evidence indicates that this rejection involves an immunologic mechanism elicited by a new transplantation antigen shared by different RadLV-induced lymphomas of W/Fu rats. All lymphomas rejected by adult syngeneic hosts had high concentrations of the RadLV cell-surface antigen, as revealed by in vitro cytotoxic and absorption tests. The concentration of this antigen was lower in a few tumors able to grow in nonirradiated adult hosts. Electron microscopy of leukemic tissues and plasma of rats with either primary or transplanted syngeneic lymphomas showed type-C virus particles in concentrations markedly higher than those previously observed in RadLV-induced lymphomas of mice. Progressive growth of 3 RadLV-induced lymphomas in rats preimmunized with the same tumor occasionally was observed. In contrast to the original lymphoma lines, sublines isolated from these outgrowths could be maintained by serial transplantation in nonirradiated adult W/Fu rats, and they were not susceptible to the rejection response of specifically preimmunized rats. They were also fully resistant to the specific cytotoxicity of RadLV antisera. After serial passage in nonirradiated recipients, in vitro absorption experiments indicated an apparently complete loss of the RadLV antigen. Extensive electron microscopic examination of the RadLV lymphoma sublines indicated that virus particles were either absent or radically decreased in concentration as compared to the original lines. One of these sublines, tested after 3 consecutive passages in preirradiated hosts, had no restoration of the antigen.