We have reported that a strain of SHR have a selective depression of T-cell functions by aging, which may be due to an early appearance of natural thymocytotoxic autoantibody and a deficiency of thymic hormone. Results presented here showed that the tumor incidence in SHR by low doses of MCA was higher than those in WKA rats with normal T-cell functions. Depression of T-cell functions in SHR could be almost completely restored by allogeneic thymus grafts or injection of extracts from vaccinia virus-infected skin tissues (NSP). When immunological restoration was achieved, generation of killer T-cells against syngeneic tumor cells in SHR was induced and activity of NK cells against K-562 cells was significantly enhanced. Effect of thymus grafts or NSP on MCA-induced primary tumors in SHR was studied. Effect of thymus grafts or NSP on MCA-induced primary tumors in SHR was studied. The tumor incidence was significantly suppressed and average latent periods were also prolonged in SHR grafted with allogeneic thymus. The administration of NSP was not effective on tumor incidence but prolonged latent periods for developments of tumors. From these results, it is suggested that the SHR is a suitable animal model for investigation of role of cell-mediated immunity in carcinogenesis.