BACKGROUND In cystic fibrosis (CF) serologic tests assume increased significance for discriminating the symptoms of allergic bronchopulmonary aspergillosis (ABPA) from those of bacterial infections. Elevated specific IgE is a sensitive indicator of ABPA, but the high prevalence of patients with CF who have simple allergy to Aspergillus fumigatus means that it lacks specificity. OBJECTIVE The aim of this study was to develop an assay to detect IgE to a single allergenic protein of A. fumigatus to increase the discrimination between ABPA and uncomplicated A. fumigatus allergy. METHODS to identify a more selective allergen, serum samples from nine patients with ABPA and five patients with allergy were used to screen A. fumigatus proteins for allergenic activity by crossed radioimmunoelectrophoresis. Molecular weights were determined by electrophoresis and immunoblotting, and carbohydrate content was analyzed by concanavalin A chromatography and endoglycosidase digestion. RESULTS The allergen identified was found to be a glycoprotein with a molecular weight of 66 kd, referred to as gp66. An ELISA was developed to measure specific IgE to gp66, and the efficacy was compared with IgE detection by using the unfractionated A. fumigatus extract on serum samples from 14 patients with CF and ABPA, 42 patients with CF and allergy but not ABPA, and 23 patients with CF without ABPA or allergy. CONCLUSIONS Use of gp66 increased the specificity of IgE analysis from 58.5% to 78.5%, whereas the sensitivity of 92.9% was maintained.