Rheumatoid arthritis (RA) is a generalized disorder characterized by chronic inflammation of peripheral joints and by involvement of many organs, including the lung parenchyma. The inflammatory infiltrates of the rheumatoid synovial membranes are associated with increased numbers of T-lymphocytes, with increased proportions of helper (OKT4 positive) T-cells and decreased percentages of suppressor/cytotoxic (OKT8 positive) T-cells, and since patients with interstitial lung disease associated with RA often have increased numbers of lymphocytes in the alveolar structures, it seemed possible that rheumatoid lung disease could also be associated with an imbalance of T-lymphocyte subpopulations. To test this hypothesis, patients with chronic interstitial lung disease and RA were evaluated by lung biopsy, gallium-67 scanning and bronchoalveolar lavage to assess the activity of the lung disorder and the T-lymphocyte subpopulations were identified with the OKT4, OKT8 and Tec T-5.9 monoclonal antibodies. The Tec T-5.9 is a recently described monoclonal antibody which recognizes a small T-cell fraction of the OKT4 positive T-lymphocytes, responsible for many helper T-cell functions, including the response to allogenic antigens and help in immunoglobulin production by B-cells. Histologic evaluation of the biopsies demonstrated active lung inflammation in all patients and gallium-67 scans showed an increased lung uptake in five of the six patients studied.(ABSTRACT TRUNCATED AT 250 WORDS)