Omalizumab is a humanized anti-IgE antibody that inhibits IgE binding to its receptors on mast cells and basophils, thus blocking the IgE-mediated release of pharmacologic mediators from these cells. Previous studies have indicated that omalizumab binds to the C epsilon 3 domain of IgE, which is the binding site of IgE receptors, but the precise epitope recognized by omalizumab is unknown. In this study, we employed the phage display peptide library technology to select peptides binding to omalizumab. A striking peptide sequence motif was recovered, which is homologous to the sequence (HLP426)-H-424 within the C epsilon 3 domain of IgE-Fc. Our results further indicated that omalizumab specifically bound to the synthesized peptide ''(421)THPHLPRALMRS(432)'' containing the (HLP426)-H-424 motif in IgE-Fc. We therefore conclude that the (HLP426)-H-424 motig is the omalizumab epitope. This epitope overlaps with the high-affinity IgE receptor-binding site, thus providing insights into the structural basis for the mechanism of action of omalizumab. (c) 2008 Elsevier Inc. All rights reserved.