The ovomucoid of Japanese quail egg white is known as a proteinase inhibitor or protein component responsible for egg allergy. In order to characterize the antigenic properties of ovomucoid in relation to its molecular structure, we have prepared two monoclonal antibodies, E9 (IgG1) and E11 (IgG1), recognizing distinct epitopes from each other. These monoclonal antibodies bound to the SDS/mercaptoethanol-treated ovomucoid, but not to the reductively carboxymethylated and pyridylethylated ovomucoid. By immunoblotting analysis of the peptic digests of ovomucoid, it was shown that E9 bound to the fragment consisting of two domains, I and II, of the ovomucoid, and that E11 reacted with the fragment containing domain III. These results indicate that the antigenic activity depends on the conformational structure of domains tightly folded by disulfide linkages. Ovomucoids from hen and duck were also recognized by both the antibodies, although having less affinity compared to the one from Japanese quail. These antibodies proved to be effective in establishing an enzyme-linked immunosorbent assay system for quantitative analysis of quail ovomucoid.