In order to determine the mechanism(s) involved in the induction of immunologic tolerance for contact sensitivity via the topical application of a chemical that sensitizes if given with adjuvant, we utilized the hapten dinitrothiocyanobenzene (DNTB). Specific immunologic tolerance to dinitrofluorobenzene (DNFB) was induced in mice by the topical application of DNTB 7 days before sensitization to DNFB. The tolerance could be abrogated if cyclophosphamide (200 mg/kg) was given 3 days before attempted sensitization. Using passive transfer studies we found that DNTB induced hapten-specific Lyt 1⁺2^- suppressor T cells. These suppressor cells prevented the induction of contact sensitivity but did not affect its expression. Lymphocyte proliferation studies, using haptenated epidermal cells as antigen, indicate that lymph node cells obtained 5 days after DNFB sensitization are far less responsive if the mice have received DNTB epicutaneously 7 days before the DNFB. Binding studies demonstrated that DNTB bound to epidermal cells at least as well as did DNFB. It is postulated that DNTB induction of suppressor cells is related to the physicochemical interaction between the hapten and antigen-presenting cells in skin.