When attempting to generate mouse monoclonal antibodies to hen's egg ovalbumin, injection of commercially purified ovalbumin resulted in monoclonal antibodies, which when assayed against commercially purified ovalbumin (Gal d I) or ovomucoid (Gal d III), appeared to be specific to both. With the use of high-performance liquid chromatography (HPLC)-repurified ovalbumin and ovomucoid in assay procedures, monoclonal antibodies generated by commercially purified ovalbumin were found to be specific for ovomucoid only. To clarify this phenomenon, mice were serially injected with commercially purified ovalbumin or HPLC-repurified ovalbumin. It was found that most of the antibody response to commercially purified ovalbumin was directed against the minor (< 1%) ovomucoid contaminant and that HPLC-repurified ovalbumin failed to produce antibodies to ovomucoid. Commercially purified ovomucoid resulted in only minimal amounts of antibodies to ovalbumin. Thus when commercially purified ovalbumin is used both for immunization and immunoassay, most of the antibodies produced are actually against the small amount of ovomucoid contaminant, and not ovalbumin. To determine whether ovomucoid is the major antigenic and allergenic egg white protein in human beings, one group of 18 children with egg allergy were skin prick tested with half-log dilutions of egg white extract and diethylaminoethyl cellulose (DEAE)-repurified ovomucoid, ovalbumin, and lysozyme. Ovomucoid mean wheal diameters were significantly greater than wheal diameters in response to ovalbumin, lysozyme, and egg white extract at the three most concentrated of five dilutions tested: 0.01, 0.03, and 0.1 mg/ml (p < 0.01). Serum ovomucoid-specific IgE and IgG antibody concentrations to DEAE-repurified ovomucoid were significantly greater than that to DEAE-repurified ovalbumin (p < 0.05). In a second study, 10 patients with egg allergy and persistent egg hypersensitivity were compared with 11 patients with egg allergy in whom clinical tolerance to egg developed. IgE antibodies to repurified ovomucoid were significantly greater in patients with persistent egg hypersensitivity compared with patients in whom clinical tolerance developed at the time of both initial and follow-up food challenges. In contrast, there were no significant differences in IgE antibody concentrations to repurified ovalbumin in either group at any time. These results suggest that ovomucoid is the immunodominant protein fraction in egg white and that the use of commercially purified ovalbumin has led to an overestimation of the dominance of ovalbumin as a major egg allergen and antigen in human beings.