The class-I and class-II molecules encoded by the major histocompatibility complex (MHC) are homologous proteins which allow cytotoxic and helper T cells to recognize foreign antigens. Recent studies have shown that the form of the antigen recognized by T cells is generally not a native protein but rather a short peptide fragment and that class-II molecules specifically bind antigenic peptides. Furthermore, the three-dimensional structure of the human MHC class-I molecule, HLA-A2, is consistent with a peptide-binding function for MHC class-I molecules. An outstanding question concerns the molecular nature and involvement of MHC-bound peptides in antigens recognized by alloreactive T cells. In this study the effects of peptides derived from HLA-A2 on cytolysis of alloreactive cytotoxic T cells (TC) cells are presented. Peptides can inhibit lysis by binding to the T cell or sensitize to lysis by binding an HLA-A2-related class-I molecule (HLA-Aw69) on the target cell. Thus, allospecific TC cells can recognize HLA-derived peptides in the context of the MHC.