Contact hypersensitivity (CHS) is a T cell-mediated response to hapten sensitization of the epidermis. The roles of CD4+ and CD8+ T cells in CHS have remained unclear, however, as studies to define either subset as the T cells mediating CHS have provided conflicting results. The goal of this study was to correlate the in vivo function of CD4+ and CD8+ T cells in CHS with the cytokines produced by each T cell population. Antibody-mediated depletion of CD4+ T cells before sensitization of BALB/c mice with 2,4-dinitrofluorobenzene (DNFB) or oxazolone (Ox) resulted in increased and prolonged CHS responses, indicating CD4+ T cells as negative regulators of the response. Depletion of CD8+ T cells resulted in low or abrogated responses, indicating CD8+ T cells as the effector cells in CHS. Sensitization with DNFB or Ox induced lymph node cell populations of CD8+ T cells producing interferon (IFN)-gamma and no interleukin (Il) 4 or Il-10, and CD4+ T cells producing Il-4 and Il-10 and no or little detectable IFN-gamma. The polarized patterns of cytokine production were stimulated by culture of hapten-primed lymph node cells either on anti-T cell receptor antibody-coated wells or with semipurified Langerhans cells isolated from hapten-sensitized mice. Stimulation of cytokine production during culture of hapten-primed CD4+ or CD8+ T cells with Langerhans cells was hapten specific and restricted to class II or class I major histocompatibility complex, respectively. The induction of the CD4+ and CD8+ T cells producing the polarized patterns of cytokines was not restricted to BALB/c mice, as cells from Ox sensitized C57B1/6 and B10.D2 mice produced the same patterns. Collectively, these results expose the induction of two polarized and functionally opposing populations of T cells by hapten sensitization to induce CHS: IFN-gamma-producing effector CD8+ T cells and Il-4/Il-10-producing CD4+ T cells that negatively regulate the response.