Pulmonary response to Aspergillus-terreus spores was studied in male New-Zealand-white-rabbits and Wistar-rats. The animals were anesthetized, the trachea was exposed, and the fungal spores were intratracheally instilled. Rabbits received 7,000,000 spores, and rats received 50,000,000. At different times after administration, animals were sacrificed, and tissue samples were prepared for histological examination. All animals were tested for the presence of precipitating serum antibodies to the Aspergillus. Phagocytosis of spores by the rat pulmonary macrophages was quantified by light microscopy. The toxicity of the spores was determined uniformly in the lungs of the rats, whereas they were most concentrated in the dorsal areas of rabbit lungs. In both species, the spores were rapidly taken up by the macrophages. In the rat lung, 42 percent were associated with the macrophages immediately after instillation. This increased to 78 percent after 30 minutes of exposure and to 100 percent after 24 hours of exposure. Granulomas formed between 48 hours and 1 week after exposure. All animals used in the study were negative for precipitating antibodies against the extract. The spores showed only slight toxicity above baseline values and were considered negative in the assay. The authors conclude that, in the absence of antibodies or spore toxicity, the pulmonary lesions (granulomas) formed cannot be the result of hypersensitivity. This model of pulmonary response to fungal spores may be of use in further characterizing the pathology associated with occupational exposure to moldy materials.