Platelet aggregation is mediated by the interaction of fibrinogen with platelet membrane glycoprotein IIb-IIIa, a member of the integrin family (integrin alpha IIb beta 3). Three different binding sites on fibrinogen for IIb-IIIa have been proposed, two RGD-containing sequences in the alpha chain and one dodecapeptide sequence at the carboxyl terminus of the gamma chain. However, recent evidence shows that mutations in either of the alpha chain sequences have no effect on platelet aggregation, whereas the substitution of a variant gamma chain (gamma‘) for the gamma chain results in a major reduction in platelet aggregation activity. The present investigation demonstrates that the gamma’ chain shows decreased binding to IIb-IIIa as measured by direct binding experiments. In addition, adhesion studies indicate that the binding of both stimulated and unstimulated platelets to immobilized fibrinogens is mediated primarily through the gamma chain carboxyl terminus. Furthermore, a peptide corresponding to the carboxyl terminus of the gamma chain inhibits fibrinogen binding and platelet adhesion, whereas a peptide corresponding to the carboxyl terminus of the gamma' chain is significantly less inhibitory. These data show that the defective platelet aggregation activity of the fibrinogen gamma' chain is due to decreased binding to platelet glycoprotein IIb-IIIa.