Protection against contact allergy begins with the collection of reliable data about the sensitizing potential of chemicals. Today, the local lymph node assay (LLNA) in mice is widely used to identify sensitizing substances. For several reasons, an in vitro assay could be preferable to animal experiments. We propose an in vitro test for the detection of a sensitizing potential of a chemical composed of a single layer of human nondifferentiating keratinocytes and of allogenic floating monocytes which are cocultured in serum-free medium in the presence of a cytokine cocktail. Within days, the coculture develops to an allergen- sensitive system consisting of activated keratinocytes and of mobile dendritic cell-related cells (DC-related cell). The sensitizing potential can be determined by analyzing the expression of the dendritic cell maturation marker CD86. For the model contact allergens tested so far [trinitrobenzenesulfonic acid (TNBS), phenylendiamine, and 4-aminoacetanilide], the strength of the reaction was in concordance with results from the LLNA. Sensitivity of the assay allowed testing at concentrations without general cytotoxicity. Thus, a differentiation between allergens and irritants was possible. Regarding cytokine secretion, the assay distinguished between the allergen TNBS and the Toll-like receptor ligand lipopolysaccharide. The coculture can be set up from cryopreserved cells. The assay is easy to perform and reproducible. Donor-variance is negligible. This in vitro assay based on a loose-fit coculture is a reasonable approach to screen for the sensitizing potential of xenobiotics and might partially replace the LLNA and other animal tests.