A screening tool for an antiplatelet agent wherein the tool is a human ADP receptor P2T.sub.AC protein, a variation functionally equivalent thereto, or a homologous protein thereof, and a screening tool for an antiplatelet agent wherein the tool is a transformant which is transformed with an expression vector comprising a DNA encoding the protein and is expressing the polypeptide are disclosed. Further, a method for detecting an ADP receptor P2T.sub.AC ligand, antagonist, or agonist by using the screening tool for an antiplatelet agent, and a method for screening an antiplatelet agent by the detecting method are disclosed.