The majority of peripheral T lymphocytes bear cell-surface antigen receptors comprised of a disulphide-linked alpha beta dimer. In an immune response, this receptor endows T cells with specificities for foreign antigenic protein fragments bound to cell surface glycoproteins encoded in the major histocompatibility complex (MHC). At a high frequency (greater than 1%), the same population of T lymphocytes responds to allogeneic MHC glycoproteins, or to differences at other genetic loci termed Mls, in conjunction with MHC. The alpha beta-antigen receptor has been implicated in alloreactivity and Mls reactivity. In fact, many monoclonal T-cell lines recognize a foreign protein fragment bound to self-MHC molecules and, in addition, recognize allogeneic MHC glycoproteins, an Mls-encoded determinant, or both. For at least one T-cell clone, a monoclonal antibody directed against the alpha beta antigen receptor has been shown to block activation induced by either antigen-bound self-MHC or by allogeneic MHC. However, it remains to be demonstrated directly that a single alpha beta receptor can mediate antigen specificity, alloreactivity and Mls reactivity, a prerequisite to understanding the structural basis of these high-frequency cross-reactivities. To address this issue we have performed transfers of receptor chain genes from a multiple-reactive T-cell clone into an unrelated host T lymphocyte. We now demonstrate definitively that the genes encoding a single alpha beta-receptor chain pair can transfer the recognition of self-MHC molecules complexed with fragments of antigen, allogeneic MHC molecules, and an Mls-encoded determinant (presumably in conjunction with MHC). In this case the transfer of antigen specificity and alloreactivity requires a specific alpha beta-receptor chain combination, whereas Mls reactivity can be transferred with the beta-chain gene alone into a recipient expressing a randomly selected alpha-chain.