In a computer modeling of transmembrane domains of human thrombin receptor, Lys-158 was found near the ligand binding site. To capture this basic residue, analogs of peptide ligand containing a series of acidic amino acids were synthesized and assayed for human platelet aggregation, and Ser-(p-F)Phe-Aad(= alpha-aminoadipic acid)-Leu-Arg-Asn-Pro-NH2 was found to be a potent antagonist. (C) 1999 Elsevier Science Ltd. All rights reserved.