The aim of this series of studies was to investigate the quality and quantity of farmers' exposure to airborne spores during the handling of hay or grain. In the beginning, the Petri dish method and later a six-stage Andersen sampler were used to collect the samples. The number of spores of mesophilic fungi, thermotolerant fungi, thermophilic actinomycetes and fungi of the Aspergillus glaucus group were determined in order to find possible causative agents of farmer's lung disease. The level of exposure varied from 10(4) cfu/m3 to 10(7) cfu/m3 (cfu = colony forming unit). In hay, fungi of the A. glaucus group usually dominated. In grain the most common moulds were Cladosporium spp. and Penicillium spp. In both hay and grain the most common thermophilic actinomycete was Thermoactinomyces vulgaris; Micropolyspora faeni was found less frequently. Silaging was found to be the best method to prevent moulding of hay. Chemicals added during baling did not satisfactorily prevent moulding of hay. For stored grain, however, the best results were obtained with propionic acid treatment. The quality and quantity of airborne spores found suggests that farm work exposes farmers to a high risk of becoming sensitized, which leads to the development of asthma or farmer's lung. Few of the methods presently available for making or storing hay and grain can satisfactorily prevent moulding. So far, use of personal dust respirators with a type P2 (previously II b) filter seems to be the only way to effectively diminish exposure to spores.