Granulomatosis caused by fungal spores of a soil saprophyte is a newly recognized pulmonary complication of intravenous drug addiction. Brown, non-budding spores were histologically identified in necrotic tissue, inside giant cells of sarcoidlike granulomata, and in the vicinity of focal angiitic lesions. The fungus was identified by culture as the dematiaceous Scopulariopsis brumptii. Cultural and histopathologic studies of lung biopsy specimens established the diagnosis. We showed precipitating antibodies to fungal antigen in the serum, prepared from the patient's isolate. Similar granulomatous pulmonary lesions were experimentally produced in mice by a single intravenous injection of spores of S. brumptii. The spores remained viable but did not show evidence of growth in the animal's tissue. Precipitating antibodies to fungal antigen and immediate wheal and late necrotizing type of skin reactions were shown in the challenged mice. The studies support the notion that pulmonary hypersensitivity to fungal spores was mediated by an Arthus'-type phenomenon.