Synthetic peptides containing the adhesion site recognition sequences present on the A alpha and gamma chains of fibrinogen were studied for their effect on the binding of endogenous platelet-von Willebrand factor (vWF) and exogenous plasma-vWf to thrombin-stimulated platelets. In agreement with previously reported data, the tetrapeptide consisting of the RGDS sequence was a more potent inhibitor of plasma-vWf binding to platelets than was the pentadecapeptide of the carboxy terminus of the fibrinogen gamma-chain (IC50 10.6 mumol/L for the RGDS tetrapeptide v 44.9 mumol/L for the gamma-chain pentadecapeptide). No apparent synergy in the inhibition of plasma-vWf binding was noted when the RGDS and gamma-chain peptides were used together (IC50 15.2 mumol/L). In contrast, the gamma-chain peptide was significantly more inhibitory than was the RGDS tetrapeptide on the binding of platelet-vWf to platelets (IC50 1.4 mumol/L for the gamma-chain pentadecapeptide v 4.5 mumol/L for the RGDS tetrapeptide, P less than .05), and there was significant synergy in the inhibition of platelet-vWf binding noted when the gamma-chain and RGDS peptides were used together (IC50 0.04 mumol/L). These results indicate that the binding of platelet-vWf to its receptor on the platelet glycoprotein IIb/IIIa complex involves both the RGDS and gamma-chain recognition sites. In contrast to the results with plasma-vWf binding, the gamma-chain recognition site appears to be more important than the RGDS recognition site in platelet- vWf binding to platelets.