Understanding the parameters involved in T cell activation has been complicated by the discovery of partial T cell agonists. Altered peptide ligands (APL) have recently shown that different subsets of T cell responses can be selectively activated by certain peptides, which define a hierarchy of T cell activation. For cytotoxic T cells, this hierarchy ranges from sensitizing target cells for lysis through proliferation to effector cell induction. The degree of TCR down-regulation mediated by APL-MHC interactions correlates well with the induction of specific T cell effector functions. This suggests that the potential agonist response induced by a given peptide occurs at different triggering thresholds. To examine the relative agonist and antagonist functions of different peptides, we have investigated the ability of lymphocytic choriomeningitis virus glycoprotein-derived APL to induce or inhibit a range of effector functions in naive CD8+ T cells. By this, we have defined a hierarchy of peptides that display a range of properties from strong agonist to no agonist function. At each level, peptides that were ranked lower in this hierarchy were able to interfere or antagonize the induction of effector functions by higher ranking peptides. We have therefore shown that this spectrum of peptides ranging from strong to no agonist function has an inverse gradient from strong antagonist to no antagonist function. Moreover, the ability of the different peptides to inhibit TCR internalization correlated with their ranking within the hierarchy. These findings support the model that antagonists are effectively preventing TCR oligomerization and functional TCR triggering.