This invention relates to a method for analyzing a sample comprising: immobilising a polypeptide to a physical support; contacting the immobilised polypeptide with a test sample which may contain an agent capable of modifying the immobilised polypeptide; contacting the immobilised polypeptide with a binding partner polypeptide, wherein the binding of this partner polypeptide to the immobilised polypeptide is at least partly dependent on the modification state of the immobilised polypeptide; and measuring the association of the binding partner polypeptide to the immobilised polypeptide.