We used ELISA to measure soluble CD8 (sCD8) in the bronchoalveolar lavage fluid (BALF) and serum of patients with summer-type hypersensitivity pneumonitis (HP). The sCD8 levels in BALF were significantly higher in the patients with summer-type HP, surpassing those found in sarcoidosis and the other pulmonary diseases studied; however, the sCD8 levels in the serum of patients with summer-type HP did not differ from the levels of the healthy controls. The numbers of CD8+ T cells were increased in the BALF of the patients with summer-type HP, and there was a correlation between the sCD8 levels and the concentrations of CD8+ T cells. Gel filtration and polyacrylamide gel electrophoresis of the fluid revealed that the anti-CD8 monoclonal antibody-reactive components in the BALF of patients with pneumonitis corresponded to a protein with a molecular weight of between 52 and 54 kDa. Soluble CD8-rich fraction purified from the BALF of patients with summer-type HP augmented in vitro lymphocytes' proliferative responses stimulated with Cryptococcus neoformans, one of the causative agents for summer-type HP. Our result suggests that soluble CD8 in the BALF may play an important role in the pathogenesis of summer-type HP.