After the resection of methylcholanthrene-induced fibrosarcomas, representing 1--2% of total body weight, "R" rats were immunized with glutaraldehyde-fixed tumor cells, irradiated sublethally [0.1419 C kg-1 (550 R)], and restored immunologically by thymus, spleen and bone marrow cells. Afterwards, 87.5% of them were able to reject a viable challenge cell graft of 1 X 10(5) cells. Sublethal irradiation alone had the same effect, reflected by 90% of rats rejecting the grafts. Five control lots were run. They showed that none of the treatments, applied to "tumor-resected" animals could provide normal animals with the same defense capacity. Results point to the decisive role of the transient presence of the tumor in the host and of the sublethal irradiation in restoration of its defense capacity. Role of partial tolerance, in producing the host's immune inhibition, and of the capacity of irradiation to abrogating it, are discussed.