Fibrinogen (Fg) mediates platelet aggregation and adhesion to artificial surfaces. The carboxyl terminus of the gamma chain of Fg (residues AGDV at gamma408-411) is known to play an exclusive role in platelet aggregation, while there is no known role for the consensus RGD sites in the Aalpha chain. In this study, we used flow cytometry to measure the coaggregation (CA) of platelets with Fg-coated beads, and investigated which domains in surface-immobilized Fg support platelet adhesion. CA of platelets with Fg-beads was nearly abolished in the presence of 4A5, a monoclonal antibody (mAb) whose epitope includes AGDV, while Z69/8, a mAb that also binds to the gamma chain carboxyl terminus but does not cover AGDV, had little effect. When beads were coated with recombinant Fg (rFg) lacking AGDV, CA was similarly abolished. In contrast, beads coated with Fg that lacked the RGDS site, supported platelet CA as did intact Fg. These results were confirmed in experiments that measured the binding of activated soluble glycoprotein IIb and IIIa (GPIIbIIIa), the platelet membrane glycoprotein complex known to be the Fg receptor, to immobilized Fg. This binding was inhibited by mAb 4A5, but not by mAb Z69/8. Binding was totally retained when beads were coated with Fg lacking RGDS, but was completely lost when beads were coated with Fg lacking AGDV. These results demonstrated that the AGDV sequence on the carboxyl terminus of the gamma chain of Fg plays an exclusive role in platelet adhesion to surface-immobilized Fg, while the carboxyl terminus of the Aalpha chain, including a consensus RGD site, is not required.