We describe antagonist peptides that specifically inhibit cytolytic activity of T cell clones and lines that express the antigen-specific receptor of CD8⁺ T lymphocyte clone 2C, which recognizes peptides in association with syngeneic (K^b) and allogeneic (L^d) MHC proteins. Addition of an antagonist peptide that can bind to K^b on 2C cells decreased the tyrosine phosphorylation of CD3 ζ chains elicited by prior exposure of the cells to an agonist peptide-K^b complex. Contrary to previous agonist-antagonist comparisons, the 2C T cell receptor had higher affinity for an antagonist peptide-K^b complex than for a weak agonist peptide-K^b complex. This difference is considered in light of evidence that antigen-specific receptor affinity values can be substantially higher when determined with the receptor on live cells than with the receptor in cell-free systems.