Clinical trials and experimental studies have demonstrated that donor T cells can play a critical role in preventing allogeneic marrow graft rejection. Results of a previous study showed that donor T cells were most effective for preventing rejection when they recognize an alloantigen expressed by recipient T cells and can cause graft-versus-host disease (GVHD). The present study examined models where marrow graft rejection can be prevented by donor T cells that do not recognize host alloantigens and cannot cause GVHD. Donor T cells prevented rejection of major histocompatibility complex (MHC) class I and ll-disparate F1 marrow in parental recipients prepared with ≥800 cGy total body irradiation (TBI) but not in those prepared with c750 cGy TBI. In recipients prepared with high TBI exposures, rejection was mediated entirely by host CD8 cells. With lower TBI exposures, rejection was mediated by host CD4 cells and CD8 cells. These observations suggested the hypothesis that donor T cells prevent rejection mediated by host effectors that recognize donor MHC class I alloantigens but do not prevent rejection mediated by host effectors that recognize donor class II alloantigens. Consistent with this hypothesis, further experiments showed that F1 donor T cells can prevent rejection of MHC class l-disparate marrow in irradiated parental recipients but have no detectable effect on rejection of MHC class ll-disparate marrow. We propose that the expression of MHC class I molecules on donor T cells makes it possible for these cells to inactivate the host response against donor class I alloantigens through a veto mechanism, whereas the absence of MHC class II molecules on murine T cells explains why these ceils cannot inactivate the host response against donor class II alloantigens. Finally, donor CD4 cells and CD8 cells were equivalently effective for preventing rejection of F1 marrow in parental recipients, suggesting that veto activity is not restricted solely to the CD8 subset of murine T cells. A veto mechanism could enable donor T cells to prevent allogeneic marrow graft rejection without causing GVHD.