A method of inhibiting platelet deposition on an arterial prosthetic surface in a patient, which method comprises:(a) administering to said patient a therapeutically effective amount of a peptide represented by Formula 1 ##STR3## wherein Z is hydrogen or C1 -C6 acyl group; X is a halogen atom; and the hydrohalic acid addition products thereof, (b) contacting circulating arterial blood of said pati