Neutralizing and protective monoclonal antibodies (mAbs) were used to fine-map the highlyconserved hemagglutinin noose epitope (H379–410, HNE) of themeasles virus. Short peptides mimicking this epitope were previouslyshown to induce virus-neutralizing antibodies [El Kasmi et al. (2000) J. Gen. , 729–735]. The epitope contains three cys-teine residues, two of which (Cys386 and Cys394) form a disulfide bridge critical for antibodybinding.