Periodontitis is an inflammatory disease caused by bacteria. In periodontitis, reactive oxygen species (ROS) are released from inflammatory cells in response to bacteria. Interleukin (IL)-8 is one of pro-inflammatory cytokines. To investigate the role of ROS in pathogenesis of periodontitis, we estimated the effect of H(2)O(2), one of ROS, on the expression of IL-8 in human periodontal ligament (PDL) cells. PDL cells were treated with H(2)O(2). IL-8 expression was determined by reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). The phosphorylation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (p38) and c-jun NH(2)-terminal kinase (JNK) was estimated by Western blotting. Treatment with H(2)O(2) at concentration of up to 250 microM increased IL-8 mRNA expression and production in a concentration-dependent manner. However, treatment with 500 microM H(2)O(2) did not increase IL-8 production. Catalase, an inhibitor of H(2)O(2), down-regulated the production of IL-8 induced by H(2)O(2). H(2)O(2) increased the phosphorylation of ERK, p38, and JNK. Pretreatment with PD98059 (ERK inhibitor), SB203580 (p38 inhibitor), or SP600125 (JNK inhibitor) decreased the IL-8 production induced by H(2)O(2). These results indicate that H(2)O(2) acts as an inducer of IL-8 secretion via activation of ERK, p38, and JNK in PDL cells. H(2)O(2) deposited in periodontal tissue during inflammation against bacteria may accelerate tissue destruction via induction of IL-8 in PDL cells.