Abstract: The present invention relates to a method for determining an extent of cyclisation of a peptide ligand displayed on a genetic display system, wherein the peptide ligand comprises a polypeptide covalently linked to a molecular scaffold at two or more amino acid residues, comprising the steps of exposing the polypeptide displayed on the genetic display system to the molecular scaffold, wherein said polypeptide comprises two or more peptide reactive groups on said two or more amino acid residues which form covalent bonds with the molecular scaffold at two or more scaffold reactive groups, to give the peptide ligand; removing unreacted molecular scaffold from the genetic display system; exposing the peptide ligand displayed on the genetic display system to a first probe, wherein the first probe binds to a first unconjugated reactive group on the peptide ligand; and measuring the first unconjugated reactive group on the peptide ligand.