The presence of percipitating antibodies has been recognized to be essential for the diagnosis of hypersensitivity pneumonitis, although the mechanism of this antibody formation is still obscure. Precipitating antibodies were examined in patients with various respiratory disease including hypersensitivity pneumonitis. Patients with hypersensitivity pneumonitis had more precipitating antibodies than patients with bronchial asthma, PIE syndrome and other fibrotic lung diseases. Furthermore, asymptomatic family members of patients with hypersensitivity pneumonitis living in the same environment were shown to have few precipitating antibodies as a normal control. The presence of polluted air containing an antigen such as fungi is nescessary but not enough for the antibody formation. These findings suggest that the high responsiveness of the immune system of the respiratory tract would be important as well as the existance of contaminated air for the pathogenesis of hypersensitivity pneumonitis.