It is well established that many cell functions are controlled by the PI-3K signaling pathway and the signaling lipid, phosphatidylinositol-3,4,5-triphosphate (PIP3). This is particularly true for mast cells which play a key regulatory role in allergy and inflammation through activation via high-affinity IgE receptors (FcεRI ) leading to activation of signaling cascades and subsequent release of histamine and other pro-inflammatory mediators. A pivotal component of this cascade is the activation of PI-3K and a rise in intracellular levels of PIP3. In this study, we developed a novel chimeric toxin that selectively binds to mast cells and which functions as a PIP3 phosphatase. Specifically, the chimeric toxin was composed of the FcεRI binding region of IgE and the active subunit of the cytolethal distending toxin, CdtB, which we have recently demonstrated to function as a PIP3 phosphatase. We demonstrate that the chimeric toxin retains PIP3 phosphatase activity and selectively binds to mast cells. Moreover, the toxin is capable of altering intracellular levels of PIP3, block antigen-induced Akt phosphorylation and degranulation. These studies provide further evidence for the pivotal role of PIP3 in regulating mast cell activation and for this signaling lipid serving as a novel target for therapeutic intervention of mast cell- mediated disease. Moreover, these studies provide evidence for the utilization of CdtB as a novel therapeutic agent for targeting the PI-3K signaling pathway.