We have re-investigated the roles of CD4 and CD8 T cell subsets in skin graft rejection across a single class I MHC disparity. Recipient mice were transplanted with skin from donors transgenic for the class I MHC molecule K b . As expected, CD8 T cells were sufficient for rapid injection; but surprisingly, CD4 T cells were also competent to do the same. Rejection was dependent on one or the other subset, since elimination of both resulted in indefinite graft survival. The possibility that alloantibody was the downstream effector of CD4 mediated rejection was excluded because CD8-depleted mice rendered B cell deficient still rejected rapidly, but T cell-depleted recipients with pre-existing high titers of alloantibody were unable to do so. In addition, if CD4 cells act to reject by recruiting and/or activating macrophages then this was not dependent on CR3, IFN-+ or TNF- § . Transplantation of skin grafts where the MHC class I disparity was at the level of passenger leukocytes only, demonstrated that transient bystander damage could occur, but that this was insufficient to result in full rejection. We surmise that for CD4 T cells to reject an MHC class I-incompatible graft it is necessary that an appropriate allogeneic peptide is processed and presented in the context of recipient MHC class II. CD4 T cells from B6 mice may fail to reject skin from MHC class I mutants because of the lack of such MHC class II-restricted presentation.