Human fibrinogen has an Arg-Gly-Asp-Ser (RGDS) sequence at residues 572- 575 of its A alpha-chain. Although RGDS-containing peptides inhibit fibrinogen binding to stimulated platelets, these peptides also inhibit platelet binding of human fibrinogen fragment X and rat fibrinogen, which lack RGDS sequences corresponding to A alpha 572–575. Thus competition between free RGD-containing peptides and internal RGDS sequence at A alpha 572–575 is not the basis for their inhibition of fibrinogen binding to platelets. Addition of a Thr to the carboxy- terminus and an Asn to the amino-terminus of the RGDS sequence, the amino acids corresponding to A alpha 576 and 571 respectively, reduced the inhibitory potency of RGDS-containing peptides by fourfold to tenfold. Arg-Gly-Asp-Phe (RGDF) corresponds to A alpha 95–98, and the RGDF peptide was an effective inhibitor of fibrinogen binding, fourfold to fivefold more potent than RGDS. Thus, local primary structure may play an important role in regulating the capacity of RGD sequences in proteins to interact with specific adhesion receptors.