The present invention relates to specific substrates for a von Willebrand factor cleaving enzyme, ADAMTS-13, as well as to diagnosis of ADAMTS-13 deficient patients, diagnostic compositions, and kits employing the substrates. Particularly preferable substrate polypeptides for ADAMTS-13 are the polypeptide which begins at amino acid 1587 and ends at amino acid 1668 of SEQ ID NO: 1 in the Sequence Listing, and the polypeptide which begins at amino acid 1596 and ends at amino acid 1668 of SEQ ID NO: 1 in the Sequence Listing. These substrate polypeptides for ADAMTS-13 have high substrate specificity and also superior quantitativeness, and a suitable size for production by recombinant methods.