This study compared the anti-platelet effect of Ac-RGDS-NH2 which is a peptide fragment from fibrinogen to Ac-RGES-NH2 in which the aspartic acid (D) of Ac-RGDS-NH2 has been replaced by glutamic acid (E). When Ac-RGDS-NH2 was infused intracoronary at concentrations of 100-400 mM, acute platelet-dependent thrombus formation in the dog coronary artery was inhibited. However, infusion of Ac-RGES-NH2 intracoronary at similar concentrations to Ac-RGDS-NH2 failed to inhibit platelet-dependent thrombus formation in the dog. Ac-RGDS-NH2 and Ac-RGES-NH2 were also tested for their ability to inhibit collagen-induced platelet aggregation in vitro. Ac-RGDS-NH2 elicited concentration-dependent inhibition of collagen-induced aggregation with no effect of Ac-RGES-NH2 on collagen-induced platelet aggregation. Thus, Ac-RGDS-NH2 is an effective antiplatelet agent after intracoronary administration in the dog and also inhibits collagen-induced platelet aggregation in vitro. Ac-RGDS-NH2 is a specific inhibitor of platelet aggregation as replacement of the aspartic acid in Ac-RGDS-NH2 with glutamic acid results in complete loss of biological activity.