Platelet integrin receptor aIIbb3 supports platelet aggregation by binding fibrinogen. The interaction between the fibrinogen C-terminal g-chain peptide composed of residues g-404-411 (GAKQAGDV) and the Arg-Gly-Asp (RGD) binding pocket on aIIbb3 is required for fibrinogen-mediated platelet aggregation, but data suggest that other ancillary binding sites on both fibrinogen and aIIbb3 may lead to higher-affinity fibrinogen binding and clot retraction. To identify additional sites, we analyzed the ability of platelets and cells expressing normal and mutant aIIbb3 to adhere to an immobilized fibrinogen plasmin fragment that lacks intact g-404-411 (‘D98’). We found the following: (1) Activated, but not unactivated, platelets adhere well to immobilized ‘D98.’ (2) Cells expressing constitutively active aIIbb3 mutants, but not cells expressing normal aIIbb3 or aVb3, adhere well to ‘D98.’ (3) Monoclonal antibodies 10E5 and 7E3 inhibit the adhesion to ‘D98’ of activated platelets and cells expressing constitutively active aIIbb3, as do small-molecule inhibitors that bind to the RGD pocket. (4) EDTA paradoxically induces normal aIIbb3 to interact with ‘D98.’ Because molecular modeling and molecular dynamics simulations suggested that the aIIb L151-D159 helix may contribute to the interaction with ‘D98,’ we studied an aIIbb3 mutant in which the aIIb 148-166 loop was swapped with the corresponding aV loop; it failed to bind to fibrinogen or ‘D98.’ Our data support a model in which conformational changes in aIIbb3 and/or fibrinogen after platelet activation and the interaction between g-404-411 and the RGD binding pocket make new ancillary sites available that support higher-affinity fibrinogen binding and clot retraction.