In blood coagulation, units of the protein fibrinogen pack together to form a fibrin clot, but a crystal structure for fibrinogen is needed to understand how this is achieved. The structure of a cove fragment (fragment D) from human fibrinogen has now been determined to 2.9 Angstrom resolution. The 86K three-chained structure consists of a coiled-coil region and two homologous globular entities oriented at approximately 130 degrees to each other, Additionally, the covalently bound dimer of fragment D, known as ’doubte-D’, was isolated from human fibrin, crystallized in the presence of a Gly-Pro-Arg-Pro-amide peptide ligand, which simulates the donor polymerization site, and its structure solved by molecular replacement with the model of fragment D.