Occurrence of molds and actinomycetes in the breathing zone of farmers during the handling of hay, straw, or grain was studied with the use of an Andersen sampler on 35 farms in Finland. On 24 farms there was a person with recently diagnosed farmer's lung disease, and on 11 farms people were free of the disease. The total spore concentration and the concentrations of the spores of Thermoactinomyces (T) vulgaris, Micropolyspora (M) faeni, and Aspergillus (A) umbrosus were statistically significantly higher on the farms of patients with farmer's lung than on the disease-free farms. The mean proportions of the spores of thermotolerant and thermophilic microbes were greater on the farms of farmer's lung patients than on the reference farms. T vulgaris was the predominant actinomycete species. Both T vulgaris and A umbrosus were found on all farms of farmer's lung patients, but M faeni on only about half of such farms. The findings match the results of previous microbiological analyses of Finnish moldy hay and serological analyses of Finnish farmer's lung patients. It seems that T vulgaris, not M faeni, may be the main causative agent of farmer's lung in Finland. The possible etiologic role of A umbrosus requires further investigation. Because the farmers often failed to identify the moldiness of the plant material in contrast to researchers, it might be possible, through training, to improve farmers' ability to identify moldiness.