Two monoclonal antibodies, anti-Ti1A and anti-Ti1B, were shown to define the clonally unique surface receptor on CT8III, a human cytolytic T lymphocyte specific for a class I major histocompatibility gene product. In the present report, this surface structure was characterized and related to the 20-kilodalton (kDa) T3 glycoprotein present on all mature human T lymphocytes. The results demonstrated that the anti-clonotypic antibodies react with an epitope on a disulfide-linked heterodimer of 49- and 43-kDa subunits exclusively expressed by CT8III. This structure is associated with T3 in the cell membrane. Similar T3-associated 49/43-kDa molecules were detected on eight additional clones, although these did not express the determinant defined by anti-Ti1A or anti-Ti1B. By probing clones of differing specificities derived from the same donor with anti-T3, it was possible to compare these T3-associated heterodimers. Biochemical analysis indicated that the 49/43-kDa structures, but not the T3 molecules themselves, had isoelectric point variability and unique peptide maps after digestion with chymotrypsin or staphylococcal protease V8. These findings support the idea that the 49/43-kDa heterodimer contains the variable region of the T cell's antigen receptor structure.