All of the T cell receptor α- and β-chain rearrangements present in a dual reactive T cell clone were characterized. This clone exhibits allelic exclusion of its β-chain genes in that only one of the two alleles is productively rearranged. Unexpectedly, it displays two productive Vα-gene rearrangements, which are both transcribed into 1.5 kb mRNA. The contribution of each of the two productive α genes to the dual recognition was analyzed by gene transfer. To this end, each of the two α genes was separately transfected with the single productively rearranged β gene. Transfer of only one of the two αβ combinations restored both allogeneic MHC recognition and self MHC-restricted antigen recognition. Thus, T cell dual recognition results from the cross-reactive recognition of an allo-MHC product by a single antigen-specific and MHC-restricted αβ T cell receptor. Furthermore, the presence of two productively rearranged α-chain genes in a T cell clone raises questions concerning the level at which allelic exclusion operates in T cells.