IgE receptors were labeled by lactoperoxidase-catalyzed surface radioiodination of rat basophilic leukemia (RBL) cells and rat peritoneal mast cells (RMC). After nonionic detergent solubilization and incubation with rat IgE, IgE-receptor complexes were immunoprecipitated using anti-rat IgE. The receptor for IgE was further purified by SDS-PAGE, the receptor peak, in the gel, was submitted to tryptic digestion, and the resulting peptides were analyzed by a 2-dimensional peptide mapping procedure. The peptides were then visualized by autoradiography. IgE receptors from different RBL cell lines exhibit slight differences in m.w., as judged by SDS-PAGE; however, no differences were seen in the tryptic peptide maps of receptors from the different RBL cell lines. In addition, receptors isolated from RMC also mapped identically, indicating that peptides responsible for the m.w. differences may not be labeled. The IgE-binding component of higher m.w., isolated by affinity chromatography on IgE-Sepharose, gave a distinct pattern of tryptic peptides that were different from the receptor. By using IgE-Sepharose and tryptic mapping, this 2nd and IgE-binding component was found on all RBL cell lines and on RMC. The membrane orientation of the receptor was analyzed by tryptic mapping. Tryptic maps obtained from IgE receptors labeled on intact cells (outside labeled only), membrane particles (inside and outside labeled), and in a detergent-solubilized form (all possible sites labeled) were similar, indicating that no other protein site was available to be labeled, in addition to those in the surface exposed binding site. Moreover, saturation of the receptor by IgE prevented its subsequent radioiodination, whether the receptor was labeled on intact cells, on membrane particles, or in a solubilized form, again indicating that no site other than the binding site could be labeled. Cumulatively, these data suggest that the IgE receptor on RBL cells is not a transmembrane protein.