Caeruloplasmin (Cp) concentration and oxidase activity have been shown to be elevated in rheumatoid arthritis (RA) and psoriatic arthritis, but normal in Reiter's syndrome, Behcet's syndrome and juvenile seronegative polyarthritis. Synovial fluid Cp was significantly depressed in comparison with serum Cp in RA. During second-line therapy in RA, Cp concentration and activity fell significantly (P less than 0.001), but the change in Cp did not correlate with plasma viscosity.