Human mast cells and basophils that express the high-affinity immunoglobulin E (IgE) receptor, Fcɛ receptor 1 (FcɛRI), have key roles in allergic diseases. FcɛRI cross-linking stimulates the release of allergic mediators. Mast cells and basophils co-express FcγRIIb, a low affinity receptor containing an immunoreceptor tyrosine-based inhibitory motif and whose co-aggregation with FcɛRI can block FcɛRI-mediated reactivity^,,. Here we designed, expressed and tested the human basophil and mast-cell inhibitory function of a novel chimeric fusion protein, whose structure is γHinge-CHγ2-CHγ3-15aa linker-CHɛ2-CHɛ3-CHɛ4. This Fcγ–Fcɛ fusion protein was expressed as the predicted 140-κD dimer that reacted with anti-human ɛ- and γ-chain specific antibodies. Fcγ–Fcɛ bound to both human FcɛRI and FcγRII. It also showed dose- and time-dependent inhibition of antigen-driven IgE-mediated histamine release from fresh human basophils sensitized with IgE directed against NIP (4-hydroxy-3-iodo-5-nitrophenylacetyl). This was associated with altered Syk signaling. The fusion protein also showed increased inhibition of human anti-NP (4-hydroxy-3-nitrophenylacetyl) and anti-dansyl IgE-mediated passive cutaneous anaphylaxis in transgenic mice expressing human FcɛRIα. Our results show that this chimeric protein is able to form complexes with both FcɛRI and FcγRII, and inhibit mast-cell and basophil function. This approach, using a Fcγ–Fcɛ fusion protein to co-aggregate FcɛRI with a receptor containing an immunoreceptor tyrosine-based inhibition motif, has therapeutic potential in IgE- and FcɛRI-mediated diseases.