The activation of dendritic cells (DC), including Langerhans cells (LC) that reside within the epidermis, is a critical event in the induction phase of allergic contact hypersensitivity. Although recently, p38 mitogen-activated protein kinase (MAPK) has been reported to play a role in the activation of DC induced by allergens, the signaling pathways involved in this process have yet to be determined. We previously found that THP-1 cells have a high capacity to induce TNF-α release and CD86, CD54, and CD40 expression following allergen treatment; reflecting in vitro allergen-induced DC activation during skin sensitization. In this study, we investigated the signaling pathways in THP-1 cells activated by two representative allergens, DNCB and NiSO₄. We found that DNCB and NiSO₄ induced phosphorylation of p38 MAPK and extracellular signal-regulated kinase (ERK). Inhibition of p38 MAPK activation selectively blocked DNCB-induced TNF-α release, but not NiSO₄-induced release. In contrast, inhibition of ERK pathways selectively suppressed NiSO₄-induced TNF-α release but not DNCB-induced release. In addition, we found that the inhibition of p38 MAPK and ERK pathways caused a selective inhibition of CD86, CD54, and/or CD40 expression following treatment with DNCB or NiSO₄. In particular, inhibition of p38 MAPK suppressed CD86, CD54, and CD40 expression induced by DNCB and CD86 expression induced by NiSO₄ while inhibition of ERK pathways suppressed CD86, CD54 and CD40 expression induced by DNCB and NiSO₄. These data indicate that both DNCB and NiSO₄ activate p38 MAPK and ERK, and thereby stimulate TNF-α release and phenotypic changes through the different signal transduction pathways.