Reaction of nitric oxide (NO.) with superoxide radical generates peroxy-nitrite, which can decompose to products that nitrate aromatic amino acids. Such nitro-aromatics may be 'markers' of NO.-dependent oxidative damage. Blood serum and synovial fluid from patients with the inflammatory joint disease rheumatoid arthritis contain 3-nitrotyrosine. By contrast, body fluids from normal subjects and patients with osteoarthritis contain no detectable 3-nitrotyrosine; much lower levels were found in serum from patients in the early stages of rheumatoid arthritis. This is evidence that NO. plays a role in joint damage in rheumatoid arthritis.