SummarySera from few Finnish patients with clinical farmers' lung react in precipitin tests with extracts of the thermophilic actinomycetes that commonly cause the disease elsewhere. Hays associated with the disease in Finland showed less evidence of spontaneous heating and contained fewer actinomycete spores than British hays. OnlyThermoactinomyces vulgariswas sometimes abundant. Some species of mesophilic fungi were more abundant than in Britain and one,Aspergillus umbrosus, reacted with most sera from farmers' lung patients in precipitin tests. A panel of antigens, including thermophilic actinomycetes,A. umbrosusand other species of theAspergillus glaucusgroup, is recommended for screening farmers' lung sera.