Gangliosides are complex glycosphingolipids that are important in many biological processes. The present study investigated the role of gangliosides in the organization of lipid rafts in RBL-2H3 mast cells and in the modulation of mast cell degranulation via FcvarepsilonRI. The role of gangliosides was examined using two ganglioside deficient cell lines (B6A4A2III-E5 and B6A4C1III-D1) as well as the parent cell line (RBL-2H3). All three cell lines examined express FcvarepsilonRI, Lyn, Syk and LAT. However, only in RBL-2H3 cells were FcvarepsilonRI, LAT and alpha-galactosyl derivatives of ganglioside GD(1b) mobilized to lipid raft domains following FcvarepsilonRI stimulation. The inhibition of glycosphingolipid synthesis in RBL-2H3 cells also resulted in a decrease in the release of beta-hexosaminidase activity after FcvarepsilonRI activation. The two mutant cell lines have a reduced release of beta-hexosaminidase activity after FcvarepsilonRI stimulation, but not after exposure to calcium ionophore. These results indicate that the alpha-galactosyl derivatives of ganglioside GD(1b) are important in the initial events of FcvarepsilonRI signaling upstream of Ca2+ influx. Since the initial signaling events occur in lipid rafts and in the mutant cell lines the rafts are disorganized, these results also suggest that these gangliosides contribute to the correct assembly of lipid rafts and are essential for mast cell activation via FcvarepsilonRI.