To study how MHC-associated self antigens may regulate the function of T cells in the periphery, we generated CD8+ T cell lines specific for a single residue variant of a self peptide. The self peptide (GAYEFTTL) was isolated from H-2-Kb class I MHC molecules immunopurified from tumor cells. CD8+ CTL lines from H-2b mice were generated against a variant peptide, pE4R, (arginine for glutamic acid at the TCR contact position 4). In short-term 51Cr-release assays, these CTL lysed H-2Kb targets that were pulsed with picomolar levels of pE4R but did not lyse target cells coated with the self peptide at micromolar levels. However, in overnight assays the CTL lysed Fas-positive target cells in the presence of nanomolar levels of the self peptide. This killing was shown to be entirely Fas/Fas ligand mediated by blocking with anti-Fas antibody and Fas-Fc chimeric molecules. While the self peptide was unable to induce serine esterase release from the CTL, it did induce secretion of IFN-gamma. By these criteria then, the unmodified self ligand served as a partial agonist for the CTL raised against a single-residue variant. CD8+ T cell lines raised by in vitro stimulation with the self peptide were likewise unable to kill self peptide-coated targets via the perforin pathway but did lyse targets via Fas. These and similar data from other groups show that self antigens (i.e., MHC/peptide complexes) may be recognized by mature peripheral T cells. The T cell population is tolerant of the self antigen in the sense that they do not respond to physiological levels of the MHC/peptide complex. However, when the level of self antigen is increased (by using synthetic peptide loading) CD8+ T cells may respond by proliferation, IFN-gamma secretion, Fas ligand upregulation, and Fas-mediated cytolysis but are still unable to respond by perforin-mediated cytolysis or granzyme release. The physiological significance of such partial activation in regulation of the immune system remains to be demonstrated.