Air pollutants including diesel exhaust particles (DEPs) have been shown to enhance allergic responses. DEPs stimulate airway epithelial cells to produce various cytokines; however, the intracellular signal transduction pathway and the involvement of reduction and oxidation (redox) control in DEP-activated signaling have not been determined. In the present study, we therefore examined the role of p38 mitogen-activated protein (MAP) kinase in DEP-induced interleukin 8 (IL-8) and RANTES production by human bronchial epithelial cells (BECs) in order to clarify the intracellular signal transduction pathway that regulates IL-8 and RANTES production. In addition, we also examined the effect of a thiol-reducing agent, N-acetylcysteine (NAC), on DEP-induced p38 MAP kinase activation and cytokine production in order to clarify the redox control mechanism in DEP-induced p38 MAP kinase activation and IL-8 and RANTES production. The results showed that DEP induced IL-8 and RANTES production and the threonine and tyrosine phosphorylation of p38 MAP kinase, reflecting the activation of p38 MAP kinase in BECs. SB 203580, as the specific inhibitor of p38 MAP kinase activity, inhibited DEP-induced IL-8 and RANTES production. NAC inhibited DEP-induced p38 MAP kinase activation and IL-8 and RANTES production. These results indicate that p38 MAP kinase plays an important role in the DEP-activated signaling pathway that regulates IL-8 and RANTES production by BECs and that the cellular redox state is critical for DEP-induced p38 MAP kinase activation leading to IL-8 and RANTES production.