We previously reported that Mls-1a B10.D2 donor preimmunization prevents the development of a lethal graft-versus-host disease (GVHD) directed against host minor histocompatibility antigens (mHAgs) in lethally irradiated (DBA/2 x B10.D2)F1 recipients (LS mice). In the same combination, the graft of T-depleted bone marrow cells also results in no GVHD (TCD BM mice). Both groups of mice exhibit a host specific tolerance. In this paper, we examined whether a graft-versus-leukemia (GVL) effect can still take place without lethal GVHD in LS and TCD BM mice. The i.v. injection of P815 tumor cells into these mice, 2-3 months after the graft, indicates an antitumor activity in LS mice but not in TCD BM mice. When the P815 cells were administered 1 day before irradiation and graft, the leukemic mortality was significantly delayed in mice reconstituted with BM and spleen cells from a preimmunized donor, but not in mice reconstituted with T cell-depleted BM. In LS mice, a subclinical GVHD develops, probably due to CTL alloreactivity against host mHAgs that is observed in vitro. Moreover, cell depletion of the donor inoculum before grafting indicates that the antitumor effect is exclusively mediated by CD8+ T cells. In summary, a beneficial GVL effect, mediated by CD8+ T cells, can be preserved without lethal GVHD.