The interactions between the constituent monomers of fibrin, the polymerized protein network that provides the structural stability of blood clots, ale frequently under stress because of the dynamic nature of blood flow Herein, the calcium dependence of the structural unfolding linked to the forced dissociation of the "A-a" knob-hole bond between fibrin monomers is reported The presence of calcium was shown to influence the incidence of the last event in the unfolding pattern characteristic of "A-a" rupture This effect, attributed to the function of the gamma 1 calcium-binding site, was found to be reversible and specific Our results indicate that binding of calcium at the gamma 1 site has no effect on the strength of the knob-hole bond prior to unfolding of the hole-containing gamma module Rather, calcium bound at the gamma 1 site makes the structure of the hole mole resilient to such forced unfolding, leading to survival of the "A-a" knob-hole bond during large: extensions of the fibrinogen molecule but at the cost of rupture of the bond at lower forces