Although the receptor with which T cells bind specific antigen can, like immunoglobulin, distinguish between antigens which differ only slightly in structure, it is unique in recognizing antigen only in conjunction with one of the self proteins of the major histocompatibility complex (MHC restriction). The receptor was identified and characterized in mouse and man by using monoclonal antibodies to receptor idiotypes, and consists of two disulphide-linked polypeptides, and acidic alpha-chain and a neutral to slightly basic beta-chain. Peptide maps have shown that, like immunoglobulin, both chains vary for receptors of different specificities. T-cell-derived cDNA clones have recently been identified in mouse and man encoding immunoglobulin-like molecules. These were identified as derived from beta-chain genes through a partial N-terminal protein sequence of the beta-chain isolated from a human T-cell tumour. We have now purified the alpha- and beta-chains of the receptor of the human T-cell leukaemia line HPB-MLT, and have determined the amino acid sequence of several tryptic peptides derived from each chain. Our results further confirm that the previously reported cDNA clones encode beta-chains. The sequence of the alpha-chain peptides identify this as another immunoglobulin-like polypeptide chain. Particularly striking was an alpha-chain peptide with high homology to the conserved portion of the immunoglobulin J segment and T-cell receptor beta-chains. Surprisingly, the alpha-chain peptides show little similarity to the sequence predicted by two overlapping putative murine alpha-chain cDNA clones.