Scopoletin (6-methoxy-7-hydroxycoumarin) is a coumarin compound and a pharmacologically active agent that has been isolated from several plant species. However, as yet there is no clear explanation of how scopoletin affects the production of inflammatory cytokine. We therefore used cells from the human mast cell line (HMC-1) to investigate this effect. Scopoletin significantly and dose-dependently inhibits the way in which phorbol 12-myristate 13-acetate (PMA) plus A23187 induces the production of inflammatory cytokines such as tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and IL-8 (P<0.05). The maximal rates at which scopoletin (0.2 mM) inhibited the production of TNF-alpha, IL-6, and IL-8 were 41.6%+/-4.2%, 71.9%+/-2.5%, and 43.0%+/-5.7%, respectively. In activated HMC-1 cells, the expression level of nuclear factor (NF)-kappaB/Rel A protein was increased in the nucleus whereas the level of NF-kappaB/Rel A in nucleus was decreased by treatment with scopoletin. Scopoletin decreased PMA plus A23187-induced luciferase activity. Scopoletin also inhibits IkappaBalpha phosphorylation and degradation in cytoplasm. These results indicate that scopoletin has a potential regulatory effect on inflammatory reactions that are mediated by mast cells.