Skin sensitization is a delayed type allergy consisting of a cellular immune reaction to small molecularweight chemicals. So far, animal test methods such as the local lymph node assay (LLNA) were usedto predict the skin sensitization potential of unknown chemicals. In line with the 3Rs concept, in vitroalternatives methods are being developed based on the early events of the skin sensitization process.One of these is the capacity of skin dendritic cells to recognize a chemical as a danger. Sincethe late nineties, L’Oréal has actively worked on the Myeloid U937 Skin Sensitization Test (MUSST)which models this early event by measuring the up-regulation of CD86 expression, a co-stimulatoryreceptor.