We studied summer-type hypersensitivity pneumonitis believed to be induced by Cryptococcus albidus in the home environments of the patients. All patients had antibodies that were reactive to Cryptococcus neoformans and Trichosporon cutaneum in sera and bronchoalveolar lavage (BAL) fluids. Cryptococcus albidus strains were isolated from 62.5% of the patient home environments. Trichosporon cutaneum was found in none of the patient homes. To study local antibody production in the lung, we cultured BAL cells to measure anti-C. neoformans and anti-T. cutaneum antibodies in the culture supernatants by the ELISA method. IgG, IgA, and IgM anti-Cryptococcus and anti-Trichosporon antibodies were found in all culture supernatants. A significant correlation was observed in antibody binding activity between Cryptococcus and Trichosporon antigen. However, the amount of IgA and IgM antibody bound to C. neoformans was significantly higher than was bound to T. cutaneum. Most anti-Cryptococcus and anti-Trichosporon antibody was absorbed by C. albidus. Our results suggest that C. albidus may be an etiologic agent in most of the cases we studied, and that IgA and IgM antibody in BAL fluid may be secreted by plasma cells in the lung.