To test the scavenging of reactive oxygen species (ROS), we added synovial fluids from patients with rheumatoid arthritis (RA) and osteoarthritis, as well as hyaluronic acid (HA) and its 2 subcomponents, D‐glucuronic acid and N‐acetyl‐D‐glucosamine, to 2 ROS‐generating systems, activated neutrophils and xanthine‐xanthine oxidase. Synovial fluid from RA patients, HA, and D‐glucuronic acid markedly decreased the O₂^‐, H₂O₂, OH‐, and chemiluminescence measured in both systems. HA and synovial fluid, which are known to be susceptible to degradation by excessive ROS in RA patients, also seem to play an active role in protecting articular tissues from oxidative damage.