Adult C57BL/6 (B6) mice exposed to fractionated irradiation or inoculated with a highly leukemogenic RadLV (A-RadLV) develop high incidence of lymphatic leukemias after a latency of 3-5 months. Inoculation of a low leukemogenic RadLV variant (D-RadLV) does not result in lymphoma development unless the animals receive a single, sublethal dose of irradiation. Whereas virus-induced primary lymphomas produce virus, express gp70 and are all immunogenic, those induced by X-rays are devoid of virus expression and are not immunogenic. A-RadLV inoculation into B6 mice is associated with early induction of virus specific, cyclophosphamide sensitive suppressor T cells which are capable of abrogating a potential antitumor immune responsiveness. D-RadLV, on the other hand, induces reactive T lymphocytes that arrest leukemogenesis unless their function is impaired by irradiation. In (B6 X BALB/c)F1 mice both A-RadLV and D-RadLV leukemogenesis depend on sublethal irradiation of the host, and both induce reactive T cells. These results suggest that (a) RadLV and X-ray induced leukemogeneses involve different etiologies. (b) Different leukemogenic treatments evoke different host responses early in latency which can either sustain or interfere with lymphoma progression.