Intraperitoneal growth of (DBA/2) P-815 tumor cells in heavily irradiated C3H mice was prevented by intravenous or intraperitoneal injection of immune, but not normal, C3H spleen cells. Tumor rejection occurred also in irradiated animals reconstituted with a pure population of immune thymus-derived (T) cells, free of bone marrow-derived (B) lymphocytes and alloantibody-forming cells. Immune spleen cells depleted of T cells were ineffective, although they contained the full complement of IgM alloantibody-forming cells. Intravenous injection of immune T cells also prevented the subcutaneous growth of P-815 cells in heavily irradiated C57BL/6 recipients. It is concluded that immune T cells played a predominant role in the rejection of allogeneic tumor cells.