The cDNAs for each of the three types of polypeptide that form the high affinity IgE receptor have been cloned and sequenced. Analysis of the predicted amino acid sequence and other data suggests that the four-chained structure (alpha beta gamma 2) contains seven transmembrane segments. The alpha chain resembles the immunoglobulin-binding chain found in other Fc receptors, but the beta and gamma chain sequences do not resemble other known proteins. (The one exception: the transmembrane segment of the gamma chains, which is homologous to the corresponding segment of the zeta chain of the CD3 complex found on T lymphocytes). Efficient expression of IgE binding by the rat receptor in COS cells was observed only when the coding sequences for each of the three chains were co-transfected. So far, only the cDNA for the human alpha chain has been successfully cloned. We attempted to express this chain by co-transfecting its cDNA with those for the rat beta and gamma chains. Surprisingly, co-transfection with the cDNA for the gamma chain was sufficient, although when the beta and gamma chains were both co-transfected, expression of alpha beta gamma oligomers was evident. Approaches being used to define by genetic manipulation the functional role of various parts of the receptor are discussed.