Abstract Antagonism of allospecific CTL by altered MHC ligands is a potential approach to specific immunomodulation of allogeneic T cell responses in acute graft rejection and graft-vs-host disease. In this study we have analyzed the capacity of peptide analogs of a natural HLA-B27-allospecific CTL epitope to antagonize direct alloreactivity. Alanine scanning demonstrated that positions 4, 5, and 7 of the peptide epitope were critical for allorecognition. A number of relatively conservative substitutions at each of these positions were then tested for their effect on allorecognition and antagonism. All substitutions at position 5 abrogated cytotoxicity. In contrast, a few changes at positions 4 and 7 were tolerated, indicating a limited flexibility of the allospecific CTL in recognition of peptide epitope variants. Most of the substitutions impairing cytotoxicity actually induced antagonism. However, whereas epitope variants with changes at positions 4 and 7 behaved as weak or intermediate antagonists, some of the variants with changes at position 5 antagonized CTL alloreactivity almost completely. The results in this study demonstrate for the first time that antagonism of direct class I-mediated alloreactivity can be achieved by variants of a natural allospecific peptide epitope.