IgG and IgA antibodies against fungus Aspergillus umbrosus have been found in the sera of patients with farmer's lung (FL) disease and healthy exposed farmers in Finland. To determine the IgE response to antigens of A. umbrosus and Candida albicans, sera from 20 patients with FL, 20 healthy farmers and 20 nonfarming controls were tested by nitrocellulose radioallergosorbent test (RAST). The values of RAST indices were low in each group and the only statistically significant difference was found between the groups of FL patients and nonfarming controls against A. umbrosus polysaccharide antigen. Individual IgE responses to polysaccharide antigens of A. umbrosus and C. albicans correlated among FL patients, however, no cross-reacting IgE antibodies could be shown. To conclude, the IgE antibody levels of A. umbrosus polysaccharide and crude antigens were low in FL patients, healthy exposed farmers and nonfarming controls. Neither polysaccharide nor crude antigen-specific IgE antibodies of A. umbrosus have any diagnostic value in FL disease.