The intensity of graft-versus-leukemia (GVL) effect was studied using several radiation-induced leukemia cell lines (designated 8027, 8313, 9107, 7929) in MHC-compatible and -incompatible allogeneic bone marrow transplantation (BMT) of leukemia-bearing C3H mice. The results indicated that BMT from MHC-incompatible allogeneic (B10, B10.D2) donors consistently improved the survival of the treated mice compared with that in syngeneic (C3H) donors. However, BMT from MHC-compatible allogeneic (B10.BR, CBA, AKR) donors failed to improve the survival of 8027-bearing recipients. On the other hand, a remarkable improvement in survival of 8313, 9107 or 7929-bearing recipients was observed in MHC-compatible allogeneic (B10.BR, AKR) BMT but there was only a marginal GVL effect in MHC-compatible BMT from CBA donors. There was no clear correlation between the intensity of GVL effect and the amount of class I MHC molecules expressed on leukemic cell lines. The activity of donor lymph node cells for the induction of lethal graft-versus-host disease (GVHD) correlated with the intensity of GVL effect induced by intact donor bone marrow. The results indicate that GVL effect against radiation-induced leukemias could be consistently induced in MHC-incompatible allogeneic BMT whereas the intensity of GVL effect induced in MHC-compatible allogeneic BMT varied among different leukemias and the donor-host strain combinations used.