Specific immune precipitation of immunoglobulin E(IgE)-receptor complexes from detergent extracts of 32P-labeled rat basophilic leukemia cells yielded a phosphoprotein of Mr approximately 35,000 on gel electrophoresis in sodium dodecyl sulfate. This phosphoprotein was shown by several criteria to be the beta chain of the receptor for IgE. Phosphorylation occurs at a serine residue (or residues) in a region (beta 2) of the beta chain that is thought to be exposed on the cytoplasmic face of the plasma membrane. Our results suggest that phosphorylation probably takes place after the insertion of the beta chain into the membrane. The IgE-binding alpha chain of the receptor and the IgE associated with it are not phosphorylated. We have so far been unable to detect any changes in the state of phosphorylation of either chain of the receptor or of IgE itself after IgE-mediated triggering of the cells.