The monoclonal antibody HM.11 recognizes modified amino acid nitrotyrosine in all different species. Nitrotyrosine is formed in tissues in presence of the active metabolite NO and is a stable end product of nitrosylation of tyrosine. Inflammation is characterized by increased nitric oxide (NO) production. NO reacts rapidly with superoxide to form peroxynitrite. At physiological pH and in the presence of transition metals, peroxynitrite undergoes heterolytic cleavage to form hydroxyl anion and nitronium ion, the latter of which nitrates protein tyrosine residues. The presence of nitrotyrosine has been detected in various inflammatory processes including atherosclerotic plaques, Amyotrophic Lateral Sclerosis (ALS) and Multiple Sclerosis (MS). Thus, the presence of nitrotyrosine on proteins can be used as a marker for peroxynitrite formation in vivo and consequently as a marker of NO-mediated tissue damage. The monoclonal antibody HM.11 recognizes nitrotyrosine, both with the free amino acid as well as with proteins containing nitrotyrosine.