1. Plasma fibronectin levels were similar in 60 healthy subjects and 88 with rheumatoid arthritis.
2. In 42 patients with rheumatoid arthritis synovial fluid fibronectin levels were significantly higher than plasma levels (P < 0.001). Intermediate fibronectin levels were found in synovial fluid from six patients with psoriatic arthritis, eight patients with osteoarthritis and seven with seronegative arthritis.
3. Plasma and synovial fluid fibronectin levels were not related to indices of inflammatory activity such as the erythrocyte sedimentation rate, the Ritchie articular index or synovial fluid cell counts. Nor did fibronectin behave as an acute-phase protein.
4. Immunofluorescent studies showed that fibronectin was adsorbed on fibrinous debris in rheumatoid arthritic joints.
5. These findings suggest that there is local production of fibronectin by the synovium and suggest that measurement of fibronectin levels in the synovial fluid may serve as an indicator of the tissue response to rheumatoid arthritis.