Hypochlorous acid is the major strong oxidant generated by human neutrophils, and it has the potential to cause much of the tissue damage that these inflammatory cells promote. It is produced from hydrogen peroxide and chloride by the heme enzyme myeloperoxidase. To unequivocally establish that hypochlorous acid contributes to inflammation, a stable and unique marker for its reaction with biomolecules needs to be identified. In this investigation we have found that reagent hypochlorous acid reacts with tyrosyl residues in small peptides and converts them to chlorotyrosine. Purified myeloperoxidase in combination with hydrogen peroxide and chloride, as well as stimulated human neutrophils, chlorinated tyrosine in the peptide Gly-Gly-Tyr-Arg. Rather than reacting directly with the aromatic ring of tyrosine, hypochlorous acid initially reacted with an amine group of the peptide to form a chloramine. The chloramine then underwent an intramolecular reaction with the tyrosyl residue to convert it to chlorotyrosine. This indicates that tyrosyl residues in proteins that are close to amine groups will be susceptible to chlorination. Peroxidases are the only enzymes capable of chlorinating an aromatic ring. Furthermore, myeloperoxidase is the only human enzyme that produces hypochlorous acid under physiological conditions. Therefore, chlorotyrosine will be a specific marker for the production of hypochlorous acid in vivo and for the involvement of myeloperoxidase in inflammatory tissue damage.