Patients with rheumatoid arthritis have altered protein patterns in their serum and synovial fluid which influences the antioxidant activity of these fluids. Rheumatoid serum has a higher antioxidant activity than control serum when ferrous and ferric ions stimulate membrane damage. The raised levels of caeruloplasmin and the lower iron saturation of transferrin contribute to these differences. When membrane damage is stimulated by a copper salt, rheumatoid serum does not show an increased antioxidant protection and has probably a lower protective activity than control serum. Attempts to damage caeruloplasmin and transferrin with oxygen radicals were unsuccessful. However, prolonged incubations with trypsin reduced the iron-binding capacity of transferrin and decreased the ferroxidase and antioxidant properties of caeruloplasmin. Copper was released from caeruloplasmin under these conditions.