Several years ago this laboratory presented evidence that SBP is a dimer composed of two subunits having similar molecular weights. The question of whether or not these subunits are identical and therefore products of a single gene remained unanswered. We now report that the two polypeptide chains are identical and that SBP is a homodimer. The experimental approach was to reduce and [14C]alkylate cystine residues in human SBP, digest the product with trypsin or cyanogen bromide and determine the number of unique amino acid sequences around each [14C]carboxymethylcysteine residue. Only four unique sequences were found when all the radioactive peptides were analyzed. Since there are eight half-cystine residues per dimer, the results support a homodimeric structure.