The pathogenesis of summer-type hypersensitivity pneumonitis, which often occurs in Japan, was examined by analysing the cell profile, especially the lymphocyte sub-populations, of bronchoalveolar lavage fluid from these patients: twenty-two normal volunteers and fourteen patients with localized lung cancer as controls. Lymphocyte sub-populations were determined by the micro-testplate method. In the bronchial fluid of the summer hypersensitivity group, the total cell number was much higher (five to ten times) than in the control groups, and the percentage of lymphocytes reached 84-2 + 5.1 (mean + s.e. mean); the percentage of T lymphocytes was significantly increased (95.6 + 1.0), but that of B lymphocytes (3.2 + 0.6) was similar to that of the control groups, though the absolute numbers of B and T lymphocytes were higher than in the control groups. In the peripheral blood of the summer hypersensitivity group, the percentage of B lymphocytes was significantly higher than that found in the normal volunteers, but that of T lymphocytes was not increased. Cellular changes in bronchial fluid were more evident than changes seen by X-ray examination and are considered to be a good parameter of the severity of hypersensitivity pneumonitis. It is considered that cell-mediated immunity as well as the Arthus reaction may be intimately related to the pathogenesis of summer-type hypersensitivity pneumonitis.