The cDNA of Aspergillus fumigatus encoding an allergen was cloned and expressed in Escherichia coli. The 987 bp long cDNA clone expressed a recombinant protein Asp f II of 34 kd. This protein exhibited binding to immunoglobulin E (IgE) in the serum samples from patients with allergic bronchopulmonary aspergillosis (ABPA). The patients with ABPA and central bronchiectasis demonstrated high levels of serum IgE antibodies, whereas patients with ABPA without central bronchiectasis, patients with asthma and Aspergillus skin test reactivity but no evidence of ABPA, and patients with aspergilloma showed only low levels of IgE antibody to Asp f II. In two-dimensional electrophoresis, a native antigen electroeluted from an A. fumigatus culture filtrate antigen preparation showed an isoelectric point and molecular weight similar to that of Asp f II. In a competitive enzyme-linked immunosorbent assay (ELISA), the IgE antibody reactivity of Asp f II with patient serum samples could be significantly inhibited by culture filtrate antigens of A. fumigatus. These results indicate that Asp f II has immunologic reactivities comparable to those of native A. fumigatus antigens. The recombinant Asp f II can be expressed in E. coli in large quantities and should prove useful as a standardized allergen for sensitive and specific immunodiagnosis of ABPA, especially in patients with central bronchiectasis.