Electron microscopy of rotary-shadowed fibrinogen demonstrates that the molecules modified for crystallization by limited cleavage with a bacterial protease retain the major features of the native structure. This evidence, together with image processing and x-ray analysis of the crystals and of fibrin, has been used to develop a three-dimensional low resolution model for the molecule. The data indicate that the two large end domains of the molecule would be composed of the carboxyl-terminus of the B beta chain (proximal) and gamma chain (distal), respectively; the carboxyl-terminus of the A alpha chain would fold back to form an additional central domain. On this basis, the carboxyl-terminal region of each of the three chains of fibrinogen is folded independently into a globular domain.