Contact hypersensitivity (CHS) is a T-cell-mediated skin inflammatory response. It is controversial whether CD4(+) T cells play an enhancing or regulatory role in the pathogenesis of CHS. Because interleukin (IL)-16 is a chemoattractant cytokine for CD4-expressing cells, we investigated the involvement of IL-16 in the CHS reaction. IL-16 production was induced in the epidermis and dermis during the elicitation phase of the CHS response with trinitrochlorobenzene. In the sensitization phase, the single application of haptens such as trinitrochlorobenzene and oxazolone also induced IL-16, whereas primary irritants or vehicle control did not. IL-16 was produced mainly by CD11c-negative cells in the epidermis during the elicitation phase. Furthermore, treatment of sensitized mice with anti-IL-16 neutralizing MoAb enhanced the ear swelling and reduced the number of infiltrating CD4(+) T cells. These data indicate that IL-16 plays a role in CHS, whereby IL-16 induces CD4(+) T cells and these CD4(+) T cells subsequently exhibit down-regulating properties.