Manifestation of graft-versus-leukemia (GVL) effect in MHC-compatible and-incompatible, allogeneic bone marrow transplantation and the roles of T cell subsets contaminated in the donor bone marrow were studied using radiation-induced leukemia-bearing C3H mice maintained under specific-pathogen-free (SPF) condition. The results indicated that BMT from MHC-incompatible allogeneic (BIO) donor significantly improved the survival of the treated mice as compared with that from syngeneic (C3H) donor. When donor (BIO) bone marrow cells were treated with either anti-Thy 1.2 or anti-Lyt 2.2 antibody plus complement prior to BMT, a beneficial GVL effect was completely abolished. On the other hand, BMT from MHC-compatible allogeneic donors (B10. BR, CBA, AKR) failed to show an improvement in survival. However, intentional enhancement of GVH reaction by preimmunization of B10. BR donor mice with a relatively small number (10 4∼ 10 5) of C3H spleen cells or by an addition of B10. BR lymph node cells to the donor bone marrow resulted in a significant improvement in survival. The depletion of all T cells completely abrogated the GVL effect, while the depletion of either Lyt 2+ or L3T4+ T cells from donor (B10. BR) bone marrow resulted in only partial, if any, abrogation of GVL effect. The results indicate that GVL effect observed in leukemic mice treated with allogeneic BMT from MHC-compatible (B10. BR) and-incompatible (BIO) donors was totally dependent on T cells contaminated in the donor bone marrow, and suggest that the roles of T cell subsets in the induction of GVL effect were different between MHC-compatible (B10. BR) and-incompatible (BIO), allogeneic BMT.