The T3/T-cell receptor complex on the surface of human thymus-derived lymphocytes consists of four glycoproteins: the alpha-chain of relative molecular mass (Mr) 40,000-50,000 (40-50K), the beta-chain (37-45K); the gamma-chain (25K) and the delta-chain (20K). The T3 alpha- and beta-chains have been identified as clonotypic T-cell receptors, but functionally the T3/T-cell receptor chains seem to form a single complex: monoclonal antibodies directed at the 20K T3 components are mitogenic for normal human T lymphocytes and, at higher concentrations, anti-clonotypic and anti-20K reagents block T-cell function. Recently, Zanders et al. showed that incubation of human T-helper clones with high concentrations of antigen abolishes antigen-specific proliferation and induces disappearance of T3 from the cell surface. Thus, the T3/T-cell receptor complex consists of two variable subunits, the T3 alpha- and beta-chains, which interact with antigen and the monomorphic 20K/25K T3 chains. Recently, the existence of a fifth polypeptide chain, the unglycosylated T3 epsilon-chain, has been postulated. Here we confirm that a 20K epsilon-chain does exist. The T3 epsilon-chain differs from the T3 delta-chain in primary structure as judged by N-terminal amino acid sequencing, peptide mapping and immunoblotting with anti-T3-delta and anti-T3-epsilon antibodies. Treatment with endoglycosidase F revealed two nonglycosylated T3 delta polypeptide backbone chains (16K and 14K) with identical amino termini. Together with previous pulse-chase experiments this observation suggests that the 14K T3 polypeptide is derived from the 16K T3 precursor by proteolytic processing near the C-terminus of the molecule.