Provided is a method for characterising a polypeptide, which method comprises the steps of; (a) optionnally reducing cysteine disulphide bridges in the polypeptide to form free thiols, and capping the free thiols; (b) cleaving the polypeptide with a sequence specific cleavage reagent to form peptide fragments; (c) optionally deactivating the cleavage reagent; (d) capping one or more ε-amino groups that are present with a lysine reactive agent; (e) analysing peptide fragments by mass spectrometry to form a mass fingerprint for the polypeptide; and (f) determining the identity of the polypeptide from the mass fingerprint.