Lung tissues from patients with rheumatoid arthritis and pulmonary involvement were studied with direct immunofluorescence. Immunofluorescence was carried out with conjugated antisera to γM, γG, γA, and β1C, as well as direct staining for rheumatoid factor with conjugated aggregates of γG globulin. Striking immunofluorescence was recorded with conjugated anti-γM and conjugated heat aggregated γ-globulin to localize tissue deposition of γM rheumatoid factor. Patchy fluorescence of γG was also noted. No specific fluorescence for γA or β1C was found. Positive lupus erythematosus (LE) cell preparations and serum levels of intermediate γG complexes (11-15S) were found in a large proportion of the patients, although no correlation with the clinical or pathologic processes could be established. It is postulated that rheumatoid factor plays an intrinsic role in the pathogenesis of the lesions found in rheumatoid lung disease.