T-CELL antigen receptors (TCR) generally interact with moderate affinity with the complex formed by major histocompatibility complex (MHC) molecules and foreign peptides(1-7), MHC/TCR recognition is followed by the generation of a signal to the T cell through a monomorphic multicomponent system that includes the CD3 complex and accessory molecules such as CD4 and CD8, The interaction between the extracellular domains of MHC and TCR molecules(1-7), and the interaction of MHC and CD4/CD8 molecules(8-10), have been considered to occur independently of one another. We report here that the affinity of CD8 dimers for MHC class I molecules is independent of haplotype and peptide content, and that the affinity of the TCR for its specific ligand is enhanced through a reduced ’off’ rate in the presence of either CD8 alpha alpha homo- or CD8 alpha beta heterodimers. Moreover, CD8 seems to help recognition of the specific MHC-peptide complex either by guiding an energetically favourable docking of TCR onto MHC, or by inducing conformational changes in the MHC complex that can augment the TCR/MHC-peptide interaction. CD8 should therefore be considered as an active participant in the T-cell recognition complex, rather than simply as an accessory molecule.