Of several dinitrobenzenes tested, 2,4-dinitrothiocyanatebenzene (DNTB) was found to be the only one that did not induce contact sensitivity when applied to the guinea pig ear epicutaneously, but when applied epicutaneously it induced tolerance to 2,4-dinitrofluorobenzene (DNFB). The manner in which DNFB, DNTB, and other dinitrobenzene compounds conjugated in vitro to soluble proteins, at physiologic pH, was examined. By measuring the free amino and sulfydryl radicals in the protein before and after conjugation, it was possible to determine to which groups the hapten was bound. It was found that although all the haptens bound to the free sulfydryl groups, DNTB was the only one that did not bind to amino groups. It is suggested that to be an epicutaenous tolerizer, as opposed to sensitizer, a hapten should bind to sulfydryl groups exclusively. It is hoped that a search for agents binding in a similar manner will reveal epicutaneous tolerizers for important industrial sensitizers.