Allelic exclusion of T cell receptor (TCR) genes is incomplete: a significant percentage (10–30%) of normal human and mouse peripheral T cells express two surface TCR α chains, and a small percentage of peripheral human T cells have been reported to express two surface TCR β chains. A proportion of thymocytes in TCR transgenic mice rearrange endogenous T cell receptor genes, and peripheral T cells with two TCR α chains, transgenic and endogenous, have been reported. T cell clones with more than a single TCR heterodimer on their surface might be expected to show specificity for more than one cognate antigen: we report here a T cell clone with dual antigen specificity, isolated from an F5 TCR influenza nucleoprotein (NP 366–374/D^b)‐specific transgenic female mouse which had rejected an H‐2‐matched male skin graft. It was selected in vitro by stimulation with male H‐2^b spleen cells in the absence of the NP366–374 peptide but has specificity for both H‐Y/D^b and NP366–374. This contrasted with the single NP366–374/D^b specificity shown by a control clone isolated from a Rag1–/– F5 mouse. The dual antigen specificity was associated with the rearrangement of endogenous TCR genes and cell surface expression of these as well as the TCR transgene.