The interactions of platelets with fibrinogen mediate a variety of responses including adhesion, platelet aggregation and fibrin clot retraction. While it was assumed that interactions of the platelet integrin _IIb3 with the AGDV sequence in the C-domain of fibrinogen and/or RGD sites in the A chains are involved in clot retraction and adhesion, recent data demonstrated that fibrinogen lacking these sites still supported clot retraction. These findings suggested that an unknown site in fibrinogen and/or other integrins participate in clot retraction. Here we have identified a sequence within C that mediates binding of fibrinogen to platelets. Synthetic peptide duplicating the 365-383 sequence in C, designated P3, efficiently inhibited clot retraction in a dose-dependent manner. Furthermore, P3 supported platelet adhesion and was an effective inhibitor of platelet adhesion to fibrinogen fragments. Analysis of overlapping peptides spanning P3 and mutant recombinant C-domains demonstrated that the P3 activity is contained primarily within 370-383. Integrins _IIb3 and ₅₁ were implicated in recognition of P3, as platelet adhesion to the peptide was blocked by function-blocking mAbs against these receptors. Direct evidence that _IIb3 and ₅₁ bind P3 was obtained by selective capture of these integrins from platelet lysates using a P3-affinity matrix. Thus, these data suggest that the P3 sequence in the C-domain of fibrinogen defines a previously unknown recognition specificity of _IIb3 and ₅₁ and may function as a binding site for these integrins.