The relative roles of CD8, TCR:pepMHC affinity, and TCR:pepMHC dissociation rate in T cell activation have remained controversial. To determine the relationships among these factors, we used T cells transfected with normal and in vitro engineered αβ TCRs, in the presence or absence of CD8. The TCRs exhibited a wide range of affinities (K_D values of 80 μM to 5 nM). T cells with the highest affinity TCRs were efficiently stimulated by peptide, with or without CD8. In contrast, CD8 was required for T cells that expressed TCRs with affinities typical of syngeneic reactions (K_D values above ∼3 μM). The results suggest that virtually all normal syngeneic interactions require CD8, which enhances peptide sensitivity by one million-fold or more.