Kamebakaurin (KA), isolated from Rabdosia excisa, has been reported to inhibit the productions of pro-inflammatory mediators. Mast cells are known to have a central role in allergic and inflammatory diseases. In this study, we investigated the effect of KA on the release of inflammatory mediators using mouse bone marrow-derived mast cells (BMMCs). The BMMCs were incubated without or with KA. The cells were stained with trypan blue to evaluate cell viability. The IgE-sensitized cells were activated by IgE cross-linking using dinitrophenyl-human serum albumin following incubation without or with KA. After antigen challenge, degranulation was evaluated by β-hexosaminidase assay, histamine was quantified by HPLC, and leukotriene B₄ (LTB₄) was assayed by ELISA. The phosphorylation of Syk and Gab2, known as the signal transduction molecules downstream of FcεRI, were evaluated by Western blot analysis. The 30 μM or less concentrations KA treated did not affect cell viability. We next investigated the effect of 30 μM or less concentrations of KA in the release of chemical mediators. The 10 or 30 µM KA significantly reduced degranulation and LTB₄ production. In addition, the exposure of these KA concentrations also tended to decrease histamine release. Further, KA decreased the phosphorylation of Syk and Gab2. These findings indicate KA blocks mast cells activation without affecting the cell viability through the inhibition of Syk and/or Gab2 phosphorylation, and KA may have the potential to become a prototype for anti-allergic and anti-inflammatory drug.