We synthesized peptides of the general formula Argn, Lysn, and (Lys-Arg)n. These agents inhibited the ristocetin-mediated binding of vWF to GPIb and the binding of asialo-vWF to platelets. This inhibitory activity was proportional to the number of lysine and/or arginine residues/molecules present. Peptides to which the sequence of Arg-Gly-Asp-Val (RGDV) had been added at the carboxy-terminus of (Lys-Arg)n, Lysn, or Argn also inhibited vWF binding. Peptides with an RGDV sequence were found to block the binding of 125I-fibrinogen to ADP-stimulated platelets. These findings indicate that the general formulae (Lys-Arg)n, Lysn, and Argn with an RGDV sequence inhibit the binding of fibrinogen to activated platelets as well as the binding of vWF to GPIb. Thus, these peptides may behave as bifunctional antiplatelet agents.