A model of acute farmer’s lung (FL) was successfully developed with Thermoactinomyces vulgaris isolated from hay in the region where a patient with FL was living. 117 rabbits were divided into 5 groups, which were given 1 or 3 intratracheal injections of 50 mg or 5 mg spores suspension respectively. After the last injection the animals were sacrificed at different intervals and the lungs were removed. Specimens were submitted to routine histologic examination and also to immunofluorescent and ultrastructural studies. Morphologic studies revealed that the intratracheal inoculation of rabbits with the spores produced a mononuclear interstitial pneumonitis similar to that seen in man. Electron microscopy showed spores in the cytoplasm of phagocytes. Immunofluorescent examination revealed deposition of IgG in the pulmonary lesions. The present study confirms that the pulmonary lesions are reversible in acute stage and also suggests that the spores not only have a direct pathogenetic effect but also can give rise to typeI and type IV hypersensitivity reactions in inducing FL.