The role of self-peptides in shaping the T cell receptor (TCR) repertoire remains to be established. While TCR reactive to certain self-peptides are thought to be depleted in the thymus, the selection of TCR specificity for foreign peptide reactivity appears to require recognition of self-peptide(s) bound to the groove of thymic major histocompatibility complex (MHC) molecules. This dichotomy suggests that different TCR affinities, accessory signals, and/or different sets of self-peptides dictate the eventual fate of any given TCR-bearing clone. Recently, it has been established for several T cell epitopes that derivatives with substitutions in TCR-contact residues can antagonize the proliferation of T cell clones against the wild-type peptide antigen. Moreover, these altered peptide ligands have demonstrated activity in the positive selection of thymocytes with TCR reactive to the wild-type peptide antigen. We have investigated the specificity of T cell antagonism with step-wise substitution of self-amino acids into each nonconserved position of a 12-amino-acid foreign peptide antigen. Our data demonstrate that the ability to antagonize proliferation without competition for MHC binding is unique to the exact self-derivative, where all five of the self-substitutions are inserted. These properties may specifically allow certain self-peptides to downregulate T cell activation to the foreign ligand and/or provide a source of stimulation for immunologic memory.