In order to elucidate the role of keratinocytes (KCs) in the induction of contact sensitivity, we applied various contact sensitizers [2,4-dinitrofluorobenzene (DNFB), urushiol, 3-n-pentadecylcatechol (PDC), 4-ethoxymethylene-2-phenyloxazol-5-one (oxazolone)] and tolerizing compounds [2,4-dinitrothiocyanobenzene (DNTB), 5-methyl-3-n-pentadecyl-catechol (5-Me-PDC)] onto the earskin of non-sensitized Balb/c mice. In addition, we applied croton oil as a non-sensitizing, but stimulatory agent. Cytokine production was demonstrated by Northern blot hybridization of the total cellular RNA extracted from epidermal cells depleted by Langerhans cells and Thy 1+ dendritic cells using radiolabeled DNA probes encoding for the murine cytokines IL-1 alpha, -2, -3, -4, TNF alpha, IFN tau, GM-CSF and G-CSF. From all cytokines tested, TNF alpha and IL-1 alpha were markedly increased upon in vivo stimulation with contact sensitizers and also after application of croton oil. Both light and electron microscopic immunostaining with a polyclonal and monoclonal antibody demonstrated the presence of TNF alpha in the epidermis. This staining was most pronounced in KCs of the suprabasal epidermis upon application of contact sensitizers or croton oil, but not with tolerizing analogues. Using a functional assay significantly more TNF alpha was found in the supernatants of KCs treated in vitro with DNFB or LPS than with DNTB. GM-CSF was found in untreated epidermis as well as in stimulated cells. The results suggest that the sensitizing properties of contact sensitizers may partly be dependent on their ability to induce proinflammatory mediators. The induction and release of TNF alpha and IL-1 alpha in KCs by contact sensitizers may play an important role in the early response to immunogenic or inflammatory signals in vivo, whereby tolerance induction seems to be less dependent on these cytokines.