Superoxide dismutase(SOD), catalase and glutathione peroxidase and the levels of lipid peroxide were assayed in erythrocytes and synovial fluid isolated from 17 patients with osteoarthritis of the knee joints and 7 with healthy knee joints as a control groups. In the erythrocytes, SOD, catalase and glutathione peroxidase activities were significantly increased in osteoarthritis compared with normal control groups, but the changes of malonyldialdehyde level was not significant. The activity of SOD in synovial fluid was significantly decreased in osteoarthritis compared with normal control groups, but catalase activity was significantly increased in synovial fluid of osteoarthritis. This result suggested that the increament of antioxidant enzymes in erythrocytes were probably due to increased production of oxygen radicals in osteoarthritis. In osteoarthritis, knee joints might be injured more easily by oxygen radicals because of decreased activity of SOD in synovial fluid of osteoarthritis.