Sir:- Contact hyypersensitivity (CHS; syn: contact allergy, contact dermatitis, contact eczema) is induced when chemicals (haptens) capable of binding covalently to autologous proteins are applied to the skin.(1)The hapten-protein complexes formed in this way are immunogenic and elicit the immune response underlying CHS. CHS is a form of delayed hypersensitivity. Exposure of a primed animal to the sensitising hapten results in erythema and induration at the site of skin contact, the response being maxima1 after 24 hours.