Carotenoids have been demonstrated to possess antioxidative and anti-inflammatory effects. However, there is no report that the effects of carotenoids on degranulation of mast cell is critical for type I allergy. In this study, we focused on the effect of carotenoids on antigen-induced degranulation of mast cells. Fucoxanthin, astaxanthin, zeaxanthin, and -carotene significantly inhibited the antigen-induced release of -hexosaminidase in rat basophilic leukemia 2H3 cells and mouse bone marrow-derived mast cells. Those carotenoids also inhibited antigen-induced aggregation of the high affinity IgE receptor (Fc⑀RI), which is the most upstream of the degranulating signals of mast cells. Furthermore, carotenoids inhibited Fc⑀RI-mediated intracellular signaling, such as phosphorylation of Lyn kinase and Fyn kinase. It suggests that the inhibitory effect of carotenoids on the degranulation of mast cells were mainly due to suppressing the aggregation of Fc⑀RI followed by intracellular signaling. In addition, those carotenoids inhibited antigen-induced translocation of Fc⑀RI to lipid rafts, which are known as platforms of the aggregation of Fc⑀RI. We assume that carotenoids may modulate the function of lipid rafts and inhibit the translocation of Fc⑀RI to lipid rafts. This is the first report that focused on the aggregation of Fc⑀RI to investigate the mechanism of the inhibitory effects on the degranulation of mast cells and evaluated the functional activity of carotenoids associated with lipid rafts.Mast cells play pivotal roles in inflammation and immediatetype allergic reactions by secreting biologically active substances including histamine, eicosanoids, proteolytic enzymes, cytokines, and chemokines. The antigen-induced aggregation of the high affinity IgE receptor (Fc⑀RI) 2 expressed on the cell surface triggers the degranulation of mast cells. Fc⑀RI has a tetrameric structure comprised of an IgE binding ␣-chain, a -chain, and a disulfide-linked ␥-chain dimer (1). The aggregation of Fc⑀RI by means of multivalent antigen-IgE complexes activates cytosolic Src protein-tyrosine kinases, such as Fyn and Lyn, which then regulate the activation of mast cells (2). Fyn kinase plays a key role in mast cell degranulation and in cytokine production by regulating Gab2 and phosphatidylinositol 3-kinase (3). Phosphorylated Lyn activates immunoreceptor tyrosine-based activation motifs of the -and ␥-chains, and the phosphorylated immunoreceptor tyrosine-based activation motifs of the ␥-chain phosphorylate Syk kinase. Thereafter, a number of other signaling and adaptor molecules, such as phospholipase C␥ and protein kinase C (PKC), are phosphorylated (4). Phospholipase C␥ catalyzes the generation both of inositol 1,4,5-trisphosphate and diacylglycerol. Inositol 1,4,5-trisphosphate is an inducer of intracellular Ca 2ϩ mobilization, which is critical for degranulation, and diacylglycerol is an activator of PKC (5). Activated PKC is translocated from the cytosol to the plasma membrane fraction. PKC regulates many functions of ma...