The Aspergillus fumigatus-specific immunoglobulin class and subclasses were evaluated in the sera of patients with allergic bronchopulmonary aspergillosis, aspergilloma, patients with asthma and immediate wheal-and-flare skin test reactivity to Aspergillus antigens, and normal controls. A sensitive sandwich enzyme immunoassay using a biotin-avidin amplification system was used to detect the specific antibodies. No difference was demonstrated in the specific antibody levels against A. fumigatus between the normal controls and A. fumigatus skin test-reactive asthmatic subjects except for immunoglobulin IgD and IgG4 isotypes. Allergic bronchopulmonary aspergillosis (ABPA) sera showed significant elevation of A. fumigatus-specific antibodies of all isotypes. On the other hand, aspergilloma sera, when compared with that of asthmatic individuals, showed significant increases of IgA, IgG1, IgG2, and IgG3 antibodies. The specific antibodies belonging to both IgG1 and IgG2 subclasses were elevated in ABPA, the former predominant in 40% of cases and the latter in 60%, while in aspergilloma IgG1 was predominant. Among the initially diagnosed ABPA patients, all specific Ig isotypes were considerably elevated when compared with sera from other patients being treated with prednisone or when compared with sera from patients who became asymptomatic after treatment.