The functions of the αC domains of fibrinogen in clotting and fibrinolysis, which have long been enigmatic, were determined using recombinant fibrinogen truncated at Aα chain residue 251. Scanning electron microscopy and confocal microscopy revealed that the fibers of α251 clots were thinner and denser, with more branch points than fibers of control clots. Consistent with these results, the permeability of α251 clots was nearly half that of control clots. Together, these results suggest that in normal clot formation, the αC domains enhance lateral aggregation to produce thicker fibers. The viscoelastic properties of α251 fibrin clots differed markedly from control clots; α251 clots were much less stiff and showed more plastic deformation, indicating that interactions between the αC domains in normal clots play a major role in determining the clot's mechanical properties. Comparing factor XIIIa cross-linked α251 and control clots showed that γ chain cross-linking had a significant effect on clot stiffness. Plasmin-catalyzed lysis of α251 clots, monitored with both macroscopic and microscopic methods, was faster than lysis of control clots. In conclusion, these studies provide the first definitive evidence that the αC domains play an important role in determining the structure and biophysical properties of clots and their susceptibility to fibrinolysis.