Establishing of alternatives to animal tests is ethically desirable and gains in importance in context of new European Union regulations such as REACH. We have refined our new in vitro assay for prediction of the sensitizing potency of xenobiotics. Monocytes cocultured with primary human keratinocytes develop to a novel class of in vitro generated dendritic cells after treatment with transforming growth factor beta and Interleukin-4 in serum-free medium. These dendritic cell-related cells (DCrc) are the key players in the loose-fit coculture-based sensitization assay (LCSA). Assay duration and cytokine consumption could be cut down without impairing the assay's functionality. DCrc showed a dose-dependent upregulation of CD86 after treatment with the contact allergens 2,4,6-trinitrobenzenesulfonic acid, the prohapten isoeugenol, and alpha-hexyl cinnamic aldehyde. The metal allergens nickel and cobalt could be detected by measuring Interleukin-6 and macrophage inflammatory protein 1-beta (MIP-1beta, CCL-4) in coculture supernatants. The irritant zinc elicited no reaction. Lipopolysaccharide produced upregulation of CD86, IL-6 and MIP-1beta. Determination of tolerable concentrations of an allergen in consumer products requires a widely accepted sharp quantitative assay. Animal-based assays do not meet this requirement. The LCSA provides dose-response information, thereby allowing prediction of the relative ability of a substance to induce sensitization.