This study was designed to evaluate immunologic differences between aspergilloma (A) and allergic bronchopulmonary aspergillosis (ABPA), comparing the results to atopic and nonatopic control subjects. Humoral studies included skin tests with common inhalant antigens and Aspergillus fumigatus. Total and specific IgE and other immunologlobulin levels and serum precipitins were evaluated against A. fumigatus. Cellular immunity was studied with routine skin testing and phytohemaqglutinin-induced lymphocyte blast transformation. Antigen-induced blast transformation was also carried out with the use of serial dilutions of A. fumigatus. All patients with ABPA were atopic and had marked elevations of IgE. None of the patients were atopic and they had normal IgE levels. Immediate and late skin reactivity to A. fumigatus was low in control and A groups but high in 2 patients with ABPA. IgG antibody against A. fumigatus was generally greater in the ABPA group. Both ABPA and A had serum precipitating antibody against A. fumigatus. The atopic controls had elevated IgE levels and immediate skin test reactivity to A. fumigatus, and one also had weak serum precipitins against A. fumigatus. IgE antibody against A. fumigatus was generally higher in ABPA than A. ABPA and A patients had elevated stimulation indices (SI) to A. fumigatus. No stimulation could be detected with cells from control subjects. This study indicates that both T and B cell sensitization may play a role in the development of or as a response to aspergillus-related pulmonary disease.