IgG, IgA and IgM ELISA antibody titers against Aspergillus fumigatus were elevated in sera of patients with aspergilloma and allergic bronchopulmonary aspergillosis (ABPA), showing higher titers for the IgG antibodies compared with the IgA and IgM antibodies. No differences were found between titers of identical antibody classes in the two groups of sera. IgG and IgA ELISA titers were highly specific whereas IgM ELISA showed more unspecific binding of IgM antibodies. Antibodies, as measured by ELISA, studied after fractionation of the antigen into fractions of decreasing molecular weight, showed a preferential binding by the high molecular weight fractions. Precipitating antibodies studied in patient sera did not always correspond with the IgG ELISA titers. IgE antibody binding was observed in all fractions from Sephadex G-100 fractionated components; maximum binding was found with fractions of 28,000-60,000 daltons. The low molecular weight fractions (18,000-less than 5,000 daltons) showed less IgE binding but the quantity of this fraction was higher. The discrepancies noted between the IgG and IgA ELISA titers and the binding of IgM or IgE antibodies indicate that antigenic components may in part differ in the binding of antibody classes.