The specific antigen receptor on human and murine T lymphocytes is a heterodimer of relative molecular mass (Mr) 80,000-90,000 (80-90K) composed of two 40-50K disulphide-linked glycoprotein subunits. Peptide map analysis of the alpha- and beta-chains of receptor isolated from distinct tumour cell lines suggests the presence of both constant and variable regions. Unlike the antigen receptor on B lymphocytes (that is, surface immunoglobulin), the human T-cell antigen receptor seems to be non-covalently associated with another invariant structure recognized by monoclonal antibodies to the cell-surface antigens T3 and Leu 4 (refs 4, 5, 9, 12). Meuer et al. have demonstrated comodulation of the T3 structure and T-cell antigen receptor using anti-clonotypic and anti-T3 monoclonal antibodies. Furthermore, immunoprecipitation with anti-T3 weakly co-precipitates a small amount of the 80-90K heterodimer in certain conditions. The murine homologue of the Leu 4/T3 structure has not been identified, although Gunter et al. have suggested that Thy-1 may be the counterpart of Leu 4/T3 (ref. 13). Here we describe a Leu 4/T3-like structure, distinct from Thy-1, associated with the T-cell receptor of a murine T-lymphoma cell line.