Nitrotyrosine has been identified as a marker of inflammation and NO production. Nitrotyrosine is formed in presence of the active metabolite NO. Various pathways including the formation of peroxinitrite lead to nitrotyrosine production. Since nitrotyrosine is a stable endproduct of peroxynitrite oxidation, assesment of its plasma concentration may be useful as a marker of NO-dependent damage in vivo. Since NOx is only an indicator for enhanced NO production, protein associated nitrotyrosine might be a more suitable marker for damage induced by reactive nitrogen intermediates derived from NO. Furthermore, most proteins have a longer half life in the circulation than NOx levels. The presence of nitrotyrosine has been detected in various inflammatory processes including atherosclerotic placques, celiac disease, rheumatoid arthritis, chronic renal failure and septic shock. In normal plasma low, undetectable, levels of nitrotyrosine are present.Nitrosylation of the aminoacid tyrosine occurs both for free tyrosine and for protein bound tyrosine. The ELISA detects nitrotyrosine containing proteins. Since the assay detects a modified aminoacid the assay is useful for proteins of all species.