Unlike the closely related chemical dinitrochlorobenzene (DNCB), which is a very strong contact allergen, dichloronitrobenzene (DCNB) has been widely regarded as a non-allergen and, as such, a useful control for its strongly sensitizing counterpart. Nevertheless, it is still an organic chemical species readily capable of penetrating skin and, rather than being regarded as completely inert, it has even been suggested to react with the immune system in such a way that it induces specific tolerance to its chemical structure. We investigated whether DCNB was in reality a non-allergen, or rather merely a weak contact sensitizer. In both a rigorously conducted guinea pig maximization test and in a modified murine local lymph node assay, DCNB was demonstrated to possess weak sensitizing activity. On this basis, DCNB cannot be regarded as inert with respect to contact allergic potential, and is therefore inappropriate as a negative control in studies of skin sensitization.