1. Thiobarbituric acid (TBA)-reactive material is present in serum and knee joint synovial fluid from rheumatoid patients, consistent with lipid peroxidation occurring in vivo.
2. The amount of TBA-reactive material in synovial fluid correlates with the concentration of iron salts present as determined by the bleomycin method, presumably because iron is an important catalyst of radical reactions in vivo.
3. There appear to be significant correlations between the contents of TBA-reactive material and bleomycindetectable iron in synovial fluid and the activity of rheumatoid arthritis as assessed with a clinical index of local inflammation and with various laboratory parameters.