Four lines of transplantable tumors, WFT-1, WFT-2, WFT-3, and WFT-4, were obtained in Wistar-King-Aptekman/Mk rats after the Friend leukemogenic virus was injected at birth. Histologically, WFT-1, WFT-3, and WFT-4 were lymphosarcomas. WFT-1 showed the "starry sky" picture described in Burkitt's African lymphoma. WFT-2 appeared to be a reticulum cell sarcoma. Both virus particles and budding were observed in these tumors by electron microscopy. It was demonstrated that these tumors will proliferate in rats if conditioned by injection of the Friend virus when they are neonates, or if they are treated with immunosuppressive procedures. However, these tumors failed to grow progressively in nonconditioned isologous rats. The mechanism of these phenomena are dis cussed.