The specific IgG antibodies to thermophilic actinomycetes in the sera from patients with farmer's lung disease were quantitated by an ELISA and the results thereof were compared with those obtained by the standard double immunodiffusion assay (Ouchterlony's method). All sera from patients with farmer's lung disease were precipitin positive to Thermoactinomyces vulgaris and/or Micropolyspora faeni by the Ouchterlony's method, and revealed significantly high levels of IgG antibody activities against these antigens by ELISA. We have found 18 precipitin-positive but asymptomatic subjects in a survey of the dairy farming population of the northern area of IWATE Prefecture. Their sera were precipitin positive to antigens related to the farmer's lung disease, however IgG antibody activities as determined by ELISA to these antigens was significantly lower in the group of asymptomatics than in the group of symptomatics. Precipitin negative dairy farming controls and normal controls living in the urban area revealed low antibody activities as determined by ELISA. From these studies, we concluded that the assay of specific IgG antibody activity to thermophilic actinomyces by ELISA is useful for the diagnosis and screening of farmer's lung disease.