Sera and synovial fluids from patients with rheumatoid arthritis (RA), and with osteoarthritis (OA) as a control, were investigated for lipid peroxide level, β-glucuronidase activity, acid phosphatase activity, and α-tocopherol level. Lipid peroxide level β-glucuronidase activity and acid phosphatase activity were found to increase in synovial fluids of RA patients, suggesting that the generation of lipid peroxide in the rheumatoid joints plays a role in the liberation of lysosomal enzymes into the synovial fluids. Signiflcant correlation was demonstrated between lipid peroxide and β-glucuronidase, and or acid phosphatase in synovial fluids from RA patients. Increased α-tocopherol level in RA synovial fluids might be explained by the increased vascular permiability in the rheumatoid joints since there was no difference in α-tocopherol level between RA and OA sera. The data led us to conclude that the increase of lipid peroxide in the synovial fluids in the rheumatoid joints played an important role in lysosomal instability of either synovial lining cells or influxed polymorphonuclear leukocytes.