In the mast cell signaling pathways, the binding of immunoglobulin E (IgE) to FcRI, its high-affinity receptor, is generally thought to be a passive step. In this study, we examined the effect of IgE alone-that is, without antigen stimulation-on the degranulation in mast cells. Monomeric IgE (500 - 5000 ng/ml) alone increased cytosolic Ca²⁺ ([Ca²⁺]_i) and induced degranulation in RBL-2H3 mast cells. Monomeric IgE (5000 ng/ml) alone also increased [Ca²⁺]_i and induced degranulation in bone marrow-derived mast cells (BMMC). Interestingly, monomeric IgE (5 - 50 ng/ml) alone-concentrations too low to induce degranulation-increased filamentous actin (F-actin) content in RBL-2H3 mast cells. We next examined whether or not actin dynamics affect the IgE alone-induced RBL-2H3 mast cell activation pathways. Cytochalasin D inhibited the ability of IgE alone (50 ng/ml) to induce de novo actin assembly. In cytochalasin D-treated cells, IgE (50 ng/ml) alone increased [Ca²⁺]_i and induced degranulation. We summarize the current findings into two points. First, IgE alone increases [Ca²⁺]_i and induces degranulation in mast cells. Next, IgE, at concentrations too low to increase either [Ca²⁺]_i or degranulation, significantly induces actin assembly, which serves as a negative feedback control in the mast cell Ca²⁺ signaling and degranulaiton.