T cells potentially encounter a large number of endogenous self- peptide/MHC ligands in the thymus and the periphery. These endogenous ligands are critical to both positive and negative selection in the thymus; however, their effect on peripheral T cells has not been directly ascertained. Using the murine allelic Hb(d)(64-76)/I-E(k) self-antigen model, we have previously identified altered peptide ligands (APLs) which are able to stimulate some but not all TCP-mediated effector functions. To determine directly the effect of endogenously synthesized APL/MHC complexes on peripheral T cells, we used a TCR transgenic mouse which had reversed our normal antigen system, with Ser69 peptide now being the agonist and Hb(d)(64-76) being the APL. In this report, we show that the constitutive level of endogenous Hb(d)(64-76)/I- E(k) complexes presented by APCs in vivo is too low to affect the response of Ser69 reactive T cells. However, by increasing the number of Hb(d)(64-76)/I- E(k) complexes expressed by the APCs, TCR antagonism is observed for both primary T cells and T cell hybridomas. In addition, the level of the CD4 coreceptor expressed on T cells changes the response pattern to enthogenously presented Hb(d)(64-76)/I-E(k) ligand. These findings demonstrate that T cells are selected to ignore the constitutive levels of endogenous complexes they encounter in the periphery. T cell responses can be affected by endogenous APLs in the periphery under limited but attainable circumstances which change the efficacy of the TCR/ligand interaction. Thus, endogenous APLs can play a role in both the selection oft cells in the thymus and the responses of peripheral T cells.