A specimen of Aspergillus fumigatus was isolated from a patient with acute bronchopulmonary aspergillosis (ABPA). Cultures were allowed to propagate and were separated into spores and mycelium to greater than 95% homogeneity. Extracts of both the spores and mycelium were prepared and used for study by both dot blot analysis and immunoblotting to study the major allergens. By dot blot analysis, 22 of 22 patients with ABPA and none of 10 healthy controls reacted with mycelium when probed for both IgG and IgE anti-Aspergillus reactivity. Similar results were obtained when these extracts were separated on polyacrylamide gel electrophoresis and then probed; this included both IgG and IgE reactivity. Further, the reactivity to preparations of whole extract was absorbed with mycelium but not spores. Several distinct protein bands were detected with IgG, ranging from 30 to 110 kilodaltons (kD). In contrast, the majority of patients with ABPA, when studied for IgE reactivity, reacted only with a 70-kD protein, although 1 out of 22 patients reacted with a 30-kD protein. Subclass analysis of IgG reactivity demonstrated that 90% of reactive sera contained IgG2 and 73% IgG4 reactivity. IgG1 and IgG3 reactivity were present but less frequently detected. The use of immunoblotting will enable the identification of relevant allergens. Further, the demonstration that the allergenicity is located primarily in mycelium will allow more definitive studies of allergen isolation for studies of reactivity and, ultimately, cloning of the 70-kD protein and, finally, epitope mapping.