The Fc region of immunoglobulin E (IgE) comprising the C epsilon 3 and C epsilon 4 domains (residues 329-547) is sufficient for binding to the high-affinity IgE Fc receptor (Fc epsilon RI alpha). Three potential N-linked glycosylation sites are present within the C epsilon 3 domain. To determine the effect of the glycosylation sites on IgE Fc synthesis and on Fc epsilon RI alpha binding, site-directed mutagenesis was performed. Mutant IgE Fc constructs were expressed in COS cells and analyzed for protein synthesis and secretion, and Fc epsilon RI alpha binding activity. We find that only N371 and N394 are glycosylated, and that the residues surrounding the glycosylation site at N394 are required for Fc epsilon RI alpha binding activity.