The CD28 molecule is thought to be essential for the costimulatory signals and is required for mitogenic activation of T cells. The aim of this study was to evaluate the contribution of CD28 in contact hypersensitivity using CD28 gene-targeted (CD28 -/-) mutant mice. Contact hypersensitivity to dinitrofluorobenzene and oxazolone was significantly decreased in CD28 -/- mice compared with that in normal (C57BL/6) mice. Significant increases in IL-2 mRNA were detected in the skin after dinitrofluorobenzene challenge in normal mice, while this response was blunted in CD28 -/- mice. In addition, responses to irritant chemicals (croton oil and phenol) were also impaired in CD28 -/- mice. IL-2, IFN-gamma, and TNF-alpha mRNA levels were lower in CD28 -/- mouse skin treated with phenol. T cells from CD28 -/- mice did not bind to epidermal cells derived from normal mice, while cocultivation of T cells with epidermal cells from normal mice demonstrated a significant binding. These results indicate that the CD28 molecule is required not only for optimal induction of allergic contact dermatitis, but also for the development of irritant-induced skin inflammation. The expression of B7-1 and B7-2 molecules on epidermal cells was induced by an allergen treatment in normal and CD28 -/- mice, whereas only B7-2 was induced after irritant treatment. Lack of CD28 signaling may influence cutaneous inflammation by modulating skin cytokine profiles, possibly due to the decreased contact of T cells with other cell populations expressing ligands for CD28. In addition, the results of this study suggest that allergic contact dermatitis and irritant dermatitis have overlapping pathophysiologic mechanisms, but subtle differences exist.