Allogeneic bone marrow transplantation is a curative treatment for leukemia and lymphoma, but graft-vs-host disease (GVHD) remains a major complication. Using a GVHD protective nonmyeloablative conditioning regimen of total lymphoid irradiation and antithymocyte serum (TLI/ATS) in mice that has been recently adapted to clinical studies, we show that regulatory host NKT cells prevent the expansion and tissue inflammation induced by donor T cells, but allow retention of the killing activity of donor T cells against the BCL 1 B cell lymphoma. Whereas wild-type hosts given transplants from wild-type donors were protected against progressive tumor growth and lethal GVHD, NKT cell-deficient CD1d−/− and Jα-18−/− host mice given wild-type transplants cleared the tumor cells but died of GVHD. In contrast, wild-type hosts given transplants from CD8−/− or perforin−/− donors had progressive tumor growth without GVHD. Injection of host-type NKT cells into Jα-18−/− host mice conditioned with TLI/ATS markedly reduced the early expansion and colon injury induced by donor T cells. In conclusion, after TLI/ATS host conditioning and allogeneic bone marrow transplantation, host NKT cells can separate the proinflammatory and tumor cytolytic functions of donor T cells.