Superoxide dismutase (SOD) activity and the lipoperoxide level of synovial fluid in patients with rheumatoid arthritis (RA) were investigated. SOD activity was checked with xanthine and xanthine oxidase reaction according to Fridovich. Lipoperoxide level was checked with thiobarbituric acid reaction according to Naito. Synovial fluid SOD activity, synovial fluid leucocyte SOD activity and synovial fluid lipoperoxide level were checked in comparison with those of osteoarthritis (OA). The correlation between SOD activity and inflammatory sings such as leucocyte count, relative viscosity, lysosomal enzyme activity (acid phosphatase activity) and C-reactive protein (CRP) concentration was checked in synovial fluid from patients with RA. The correlation between lipoperoxide level and inflammatory sings was checked similarly. The results are summarized as follows: 1) Synovial fluid SOD activity in RA was higher than in OA. Synovial fluid leucocyte SOD activity was significantly higher than that of OA. 2) The synovial fluid lipoperoxide level of RA was significantly higher than that of OA. 3) A significant correlation was present between synovial fluid SOD activity and leucocyte count, lipoperoxide level, acid phosphatase activity. 4) A significant correlation was present between synovial fluid leucocyte activity and leucocyte count, lipoperoxide level, acid phosphatase activity, CRP concentration. 5) A significant correlation was present between synovial fluid lipoperoxide level and leucocyte count, acid phosphatase activity. 6) Superoxide and lipoperoxide may be an inducer of inflammation in view of RA inflammation. It is premature to pass judgement on the generally accepted idea that SOD is an antiinflammatory protein.