We have previously demonstrated that T lymphomas may express clonally specific epitopes that are carried by a T-cell-restricted, disulfide-bonded heterodimeric glycoprotein. We have used a monoclonal antibody, 124-40, to isolate the lymphoma-specific antigen and raise a xenoantiserum to the molecule. This antiserum immunoprecipitates a family of disulfide-bonded dimers from normal thymocytes and T cells, but is unreactive with B cells. Peptide maps prepared after limited proteolytic digestion indicate that the molecules from the different cell populations have homologous primary structures. Comparison of two-dimensional tryptic peptide maps indicate that, in addition to several common peptides, the molecules exhibit considerable structural heterogeneity. Taken together, these data indicate that the T-cellspecific heteroduplex has regions of constant and variable structure consistent with the properties expected for the T cell antigen receptor.