A modified fibrinogen molecule which is missing the COOH-terminal portion of the A alpha chain has been used in structural investigations of the mechanism of assembly of the fibrin clot. Brief plasmin digestion of human fibrinogen, followed by ammonium sulfate fractionation and column chromatography, yielded a highly clottable fragment X-like preparation. Molecules in this preparation contain mostly intact B beta and gamma chains, but are missing the COOH-terminal two-thirds of the A alpha chain. Clots formed by addition of thrombin to this fragment were mechanically unstable and easily dispersed. Electron microscopy showed that the clots consist mainly of a suspension of individual fibers, in contrast to clots made from native fibrinogen, which are highly branched. It appears, therefore, that a part of the COOH-terminal two-thirds of the alpha chain is necessary for branching of fibers to form a stable three-dimensional gel. Intermolecular interactions of this portion of the alpha chain are consistent with certain of its unusual features, such as its apparent existence, in part, as a single polypeptide chain and its involvement in Factor XIIIa-mediated ligation between molecules.