Fine fibrin clots and coarse and fine fibrin films (both ligated and unligated), formed by shrinkage of clots in one dimension, were examined by electron microscopy. Specimens of clots were prepared by critical point drying and by embedding and sectioning; specimens of films were prepared by embedding and sectioning only. In the fine clots, network junctions appeared to be formed by fiber segments in which two or more protofibrils were gently twisted around each other for distances of the order of 200 nm and then diverged to give trifunctional branch points. This topology appeared to be preserved in the fine films. It is proposed that the strength of the junctions is primarily provided by the twisting topology, though reinforced by non-covalent bonding involving the B sites uncovered by thrombin. In coarse films, bundles of protofibrils, lying primarily in the film plane, had diameters of 40 to 200 nm and were gently twisted around each other to form thicker cables. Uniaxial stretching, up to 100%, of either fine or coarse film before fixing caused suprisingly extensive orientation of the protofibrils or bundles. However, random orientation was recovered if a stretched ligated film was allowed to retract to its original dimensions before fixing. In a stretched coarse film sectioned perpendicular to the stretch direction, fiber bundles could be seen in cross-section; these were roughly circular with scalloped edges. The changes with stretching and recovery are discussed in relation to possible mechanisms of deformation and elastic energy storage.