The surface expression CD63 antigen in rat basophilic leukemia cells (RBL-2H3) was observed after antigen stimulation by confocal fluorescence microscopy. The surface expression of CD63 antigen reflected the degranulation in RBL-2H3 cells. Then, we did the same experiments in P815 mastocytoma cells with transfected IgE receptors. The expression was observed in P815 cells with normal IgE receptors, but not in P815 variant cells with IgE receptors which were missing a C-terminal cytoplasmic domain of beta or gamma subunit. In addition, the expression in P815 cells with normal IgE receptors was mostly blocked by the pretreatment of herbimycin A. The results suggested that tyrosine phosphorylation of the C-terminal cytoplasmic domains of beta and gamma subunits was essential for degranulation.