Human fibrinogen is a dimer composed of two identical halves. Each dimeric half contains three peptide chains (alpha, beta, and gamma) linked by disulfide bonds. The two half-molecules are joined by three disulfide bonds, one between the two alpha-chains (residue alpha-28) and two between the two gamma-chains (residues gamma-8 and gamma-9). In the absence of any difinitive experimental evidence, it has been presumed that the joined halves were aligned in a parallel orientation similar to the situation found in immunoglobulins. We have now determined that the two gamma-chains--hence, the dimeric halves--are connected in an antiparallel manner. A tryptic peptide containing gamma-chain residues 6-14 was isolated as a disulfide-linked dimer from CNBr-treated fragment E. Synthetic peptides corresponding to this sequence were prepared, from which parallel and antiparallel dimers were constructed. During the syntheses, cysteine thiol groups were protected as p-methoxybenzyl and acetamidomethyl sulfides; the peptides were dimerized by selective deprotection and disulfide bond formation. First, the p-methoxybenzyl groups were removed by liquid hydrogen fluoride and the newly exposed thiols oxidized in the presence of potassium ferricyanide. Then the monocystine compound was converted to the double-cystine product by iodolytic cleavage of the acetamidomethyl group with concomitant disulfide bond formation. This selectivity was used to prepare peptide dimers which modeled both parallel and antiparallel arrangements. The antiparallel-oriented synthetic peptide was indistinguishable from the native tryptic peptide as judged by elution from reverse-phase high-performance liquid chromatography and circular dichroism spectroscopy. The parallel-oriented synthetic peptide differed from the native material by both criteria.