Background: Although reactive oxygen species (ROS) have been produced in both mouse bone marrow-derived dendritic cells (DCs) and XS-106 DCs by contact sensitizers and irritants in previous studies, the generation of ROS in human monocyte-derived DCs (MoDCs) and their role in contact hypersensitivity (CHS) has yet to be elucidated. Objective: The purpose of this study was to determine whether contact allergens and irritants induce ROS in MoDCs and, if so, to evaluate the role of contact allergen and irritant induced-ROS in MoDCs in CHS. Methods: Production of ROS was measured by 5-(and-6)-chloromethyl-2`,7`-dichlorodihydrofluorescein diacetate (CM-H2DCFDA) assay. Surface CD86 and HLA-DR molecules were detected by flow cytometry. Protein carbonylation was detected by Western blotting. Results: ROS were produced by contact allergens such as dinitrochlorobenzene (DNCB) and thimerosal and the irritant benzalkonium chloride (BKC). DNCB-induced, but not BKC-induced, ROS increased surface CD86 and HLA-DR molecules on MoDCs and induced protein carbonylation. These changes were reduced in the presence of antioxidant N-acetyl cysteine. Conclusion: Our results suggest that DNCB-induced ROS may be different from those induced by irritant BKC. The DNCB-induced ROS may be associated with the CHSresponse, because they activate surface molecules on DCs that are important for generating immune reactions. (Ann Dermatol 22(3) 269∼278, 2010)