The high affinity receptor for IgE (Fc epsilon RI) found on mast cells and basophils is a tetrameric complex of a single alpha subunit, a single beta subunit, and two identical gamma subunits. The genes for the three subunits of mouse Fc epsilon RI have now been cloned from the mast cell line, PT18. When compared at the DNA level, the rat and mouse subunits are similarly conserved. However, at the protein level the homology between mouse and rat alpha is surprisingly low (71% identities) especially in the cytoplasmic regions (57% identities) which are of different length (25 and 20 residues, respectively). By contrast the beta and gamma are homogeneously conserved between mouse and rat (83 and 93% identities, respectively). The consensus amino acid sequence of the alpha subunit derived from three species (rat, mouse, and human) shows that the cytoplasmic tail diverges to the same extent as the leader peptide. Conversely, the transmembrane domain of the alpha is highly conserved and contains 10 consecutive residues that are identical. Comparisons between mouse Fc epsilon RI and other mouse proteins reveal regions of high homology between the alpha subunit and Fc gamma RIIa and between the gamma subunit and the zeta chain of the T cell receptor. Cells transfected with the alpha gene express the alpha subunit on their surface very inefficiently. Efficient expression is only achieved after co-transfection of the three rodent genes or of the human alpha gene together with the rodent gamma without apparent need for beta. The subunits are completely interchangeable upon transfection so that various chimeric mouse-rat-human receptors can be expressed.