Wistar rats treated with cyclophosphamide, total lymphoid irradiation (TLI), and/or cyclosporin A (CSA) develop a state of immune suppression permitting the growth of tumor xenografts. Experiments were carried out on this newly developed model to investigate the growth patterns of a mouse osteosarcoma and a human colon adenocarcinoma. The combination of cyclophosphamide and CSA permitted a limited period of growth of the mouse osteosarcoma with a tumor take rate of 66%. No takes were observed with the human adenocarcinoma. The combination of cyclophosphamide and TLI resulted in a period of immunosuppression followed by recovery of the immune status. During the period of immunosuppression, tumor xenografts showed a 100% take rate. The most efficient immunosuppression was achieved by a combination of cyclophosphamide, TLI and CSA administered on alternate days. Wistar rats subjected to this treatment showed prolonged tolerance to mouse osteosarcoma and human adenocarcinoma xenografts. There was no alteration in the tumor doubling time or histological morphology of the xenografts in the adapted host as compared with those in the donor tumors. The tumor growth curve showed a pattern of initial growth, a period of stagnation, followed by a steady but slower growth phase. The significance of the results and the advantages of the rat model described in this paper for human tumor xenotransplantation are discussed.