PROBLEM TO BE SOLVED: To provide a polypeptide used for treating the state of requiring the modulation of platelet-mediated aggregation.SOLUTION: This therapeutic polypeptide consists of a polypeptide comprising at least one single domain antibody directed to vWF, vWF A1 domain, activated vWF A1 domain, vWF A3 domain, gpIb and/or collagen, the homologues of the polypeptide and/or the functional part of the polypeptide. A method for producing the polypeptide, a method for coating devices used in medical treatments (e.g. PCTA, stenting) with the polypeptide, a method and kit for screening agents modulating the platelet-mediated aggregation, and a kit for the diagnosis of the diseases related to the platelet-mediated aggregation are also provided.