Bovine synovial fluid was degraded upon exposure to reactive oxygen species. Degradation was monitored as a decrease in relative viscosity, measured by timing the flow of 0.6 mL synovial fluid through a 1 cc syringe fitted with a 20 gauge needle. Superoxide radicals were produced by the action of xanthine oxidase on hypoxanthine. Upon addition of xanthine oxidase and hypoxanthine to synovial fluid, the viscosity decreased over time. Additions of superoxide dismutase (SOD), catalase, EDTA, DETAPAC, SOD plus catalase or Mn-salophen (an SOD mimic) diminished markedly the decrease in viscosity. These antioxidants protected synovial fluid from oxidative damage, consistent with the degradation of synovial fluid by the superoxide radical, hydrogen peroxide and the hydroxyl radical.