Human monocytic cell line THP-1 cells are used as an indicator for in vitro skin sensitization testing. Although p38 mitogen-activated protein kinases (MAPKs) and intracellular redox imbalance play crucial roles in the activation of THP-1 by skin sensitizers, the trigger of cell activation has not been identified. Therefore, we examined whether haptens induce THP-1 maturation directly or indirectly. 2,4-Dinitrochlorobenzene (DNCB), but not dinitrophenol (DNP)-conjugated bovine serum albumin or DNP-conjugated fetal bovine serum, induced CD86 expression. DNCB and nickel sulfate (NiSO₄) also induced related changes of cell-surface thiols and phosphorylation of p38 MAPK. However, DNCB is membrane-permeable, and so its direct effect may not be confined to cell membrane proteins. Next, we found that CD86 expression and macrophage inflammatory protein-1β (MIP-1β) production were augmented by the membrane-impermeable thiol blocker 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), and these changes were suppressed by an inhibitor of the p38 MAPK pathway, SB203580. Finally, we confirmed that endocytotic activity for bovine serum albumin (BSA) Alexa Fluor 488 conjugate did not affect cell-surface thiols on THP-1 cells. Thus, our data indicate that the changes of cell-surface thiols are one of the triggers of maturation, and play a key role in activation of THP-1 cells by haptens.