The role of the carbohydrate portion of the receptor for IgE in the interaction with IgE was investigated. Membrane carbohydrates of rat basophilic leukemia (RBL) cells and rat mast cells (RMC) were labeled by treating the cells with galactose oxidase followed by [3H]-NaBH4. IgE receptors were separated from detergent solubilized membranes and examined by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Pretreatment with neuraminidase markedly increased the incorporation of 3H into both the total membrane extract and into the IgE receptors. SDS-PAGE analysis demonstrated the presence of galactose in all IgE-binding components of 2 RBL cell lines and the presence of sialic acid on the major IgE-binding component. Prior saturation of the cells with IgE did not prevent the carbohydrate labeling of the receptor, although it did block the labeling of its protein part, indicating that carbohydrates are not located in the binding site. Removal of terminal sialic acid residues with neuraminidase increased the affinity of the receptor for IgE without appreciably affecting the number of receptors per cell. In order to more drastically modify the receptor carbohydrates, RBL cells were grown in the presence of Tunicamycin (TN). TN was shown to markedly inhibit the incorporation of [3H]-glucosamine into the receptor. RBL cells grown in the presence of TN expressed fewer receptors at the cell surface, as judged both by ligand binding studies and external labeling procedures. These data cumulatively suggest that the carbohydrate moieties of the receptor are not directly located in the binding site of the IgE receptor; however, the TN studies suggest that receptor carbohydrate may play a role in transport of the receptor to the plasma membrane or in its orientation thereafter.