The high-affinity receptor for immunoglobulin E, Fc_ɛ RI, is found exclusively on mast cells and basophils. When multivalent aller-gens bind to the receptor-bound IgE, the consequent aggregation of the receptors leads to the release of mediators responsible for allergic symptoms. In rodents Fc_ɛ RI is a tetrameric complex of non-covalently attached subunits: one IgE-binding α subunit, one β subunit and a dimer of disulphide-linked γ subunits¹. Com-plementary DNA encoding the α and the β subunits has recently been isolated^2–5, but expression of IgE-binding by transfected cells has not yet been achieved^2–5. Here we report the cloning of cDNA for the γ subunit, and propose a model for the αβγ₂ tetramer which accounts for many of the structural features of the receptor. The rodent receptor on the surface of COS 7 cells was expressed only when the cDNAs for all three subunits were cotransfected. Successful expression of human IgE receptors should now be possible, eventually to permit the detailed analysis of the human IgE-receptor interaction and assist the search for therapeutically effective inhibitors.