The human Cell Line Activation Test (h-CLAT) is cell-based method which identifies skin sensitizers by examining changes in the expression of cell surface markers (CD54 and CD86). Following exposure of the THP-1 human monocyte cell line to the test substance, expression levels of CD54 and CD86 are quantified by flow cytometry and compared to controls. The h-CLAT method addresses the third key event, dendritic cell activation, of the skin sensitization AOP. The method has been reviewed by EURL-ECVAM and has been recommended as a useful tool in an integrated testing strategy for identification of skin sensitizers. The assay has an OECD-approved guideline and is included among our other sensitization assays, KeratinoSensTM and DPRA.