The experiments described here examined the capacity of different microorganisms to induce allergic alveolitis. Guinea-pigs were exposed to an aerosol of pure cultures of five different organisms, four of which are common in mouldy hay, without previous injection of an adjuvant. The animals were either acutely exposed or exposed for 3 to 5 weeks, after which the numbers of different inflammatory cells in the airways were counted and histological changes in the lung parenchyma were assessed. It was seen that prolonged exposures to large numbers of spores produced a cellular infiltration in the alveolar and bronchiolar region, and gave rise to lesions resembling early granulomas. The number of lymphocytes increased in the airways. The results suggest that allergic alveolitis can be induced by inhalation of various kinds of microorganisms and that these may vary in their capacity to produce the disease.