Pulmonary sarcoidosis is a disease characterized by increased numbers of T-lymphocytes in the alveolar structures, which through the production of lymphokines modulate granuloma formation and polyclonally activate B cells to secrete immunoglobulins. The T-lymphocyte alveolitis is associated with a different expansion of various T-cell subpopulations identified by different monoclonal antibodies. Patients with active disease have increased numbers of helper T cells in the lungs, recognized by the OKT4 monoclonal antibody and decreased numbers of suppressor OKT8-positive lung T cells, whereas patients with inactive disease have increased numbers of OKT8-positive T cells and decreased numbers of OKT4-positive T cells in the lungs. Using the IgG fraction of a monoclonal antibody called 5/9, which reacts in normal subjects with approximately 30% of the OKT4-positive T-lymphocytes, it has been shown that the 5/9-positive T cells appear preferentially expanded in pulmonary sarcoidosis at sites of disease activity. To evaluate the functions of the T-cell subpopulation identified by the 5/9 monoclonal antibody in pulmonary sarcoidosis, we studied the unfractionated T-lymphocytes and the 5/9-positive and the 5/9-negative T-cell fractions in bronchoalveolar lavage of 12 patients with active lung disease. On T-cell suspensions, the spontaneous release of monocyte chemotactic factor and the polyclonal activation of autologous peripheral blood lymphocytes were determined.(ABSTRACT TRUNCATED AT 250 WORDS)