Nitrated and chlorinated proteins were found in bacteria phagocytosed by polymorphonuclear cells, suggesting a host–defense mechanism mediated by reactive nitrogen and chlorine species (Evans TJ et al. 1996; Hazen SL et al. 1996). Hypochlorous acid (HOCl) was shown to react with nitrite NO2(-) to form nitryl chloride (NO2Cl) (Eiserich JP et al. 1996). NO2Cl formation by activated human neutrophils in the presence of added NO2(-) has been also demonstrated (Eiserich JP et al. 1998). The addition of HOCl to isolated DNA or cells in the presence of NO2(-) results in increased cytosine chlorination and DNA oxidation compared with DNA or cells in the absence ofadded NO2(-) (Whiteman M et aal. 1999; Spencer JP et al. 2000). NO2Cl was also capable of nitrating, chlorinating, and dimerizing phenolic compounds such as tyrosine (Jacob JS et al. 1996; Eiserich JP et al. 1996, 1998). These studies have emphasized the potential toxic effects of nitrite and HOCl-reaction products. However, NO2(-) has also been reported to inhibit the antimicrobial activity of HOCl and MPO (van Dalen CJ et al. 2000;Marcinkiewicz J et al. 2000).