Myeloperoxidase (MPO) mediates pathogen destruction by generating the bactericidal oxidant hypochlorous acid (HOCl). Formation of this oxidant is however associated with host tissue damage and disease. MPO also utilizes HO to oxidize other substrates, and we hypothesized that mixtures of other plasma anions, including bromide (Br), iodide (I), thiocyanate (SCN) and nitrite (NO), at normal or supplemented concentrations, might modulate MPO-mediated HOCl damage. For the (pseudo)halide anions, only SCN significantly modulated HOCl formation (IC ∼33 μM), which is within the normal physiological range, as judged by damage to human plasma fibronectin or extracellular matrix preparations detected by ELISA and LC-MS. NO modulated HOCl-mediated damage, in a dose-dependent manner, at physiologically-attainable anion concentrations. However, this was accompanied by increased tyrosine and tryptophan nitration (detected by ELISA and LC-MS), and the overall extent of damage remained approximately constant. Increasing NO concentrations (0.5-20 μM) diminished HOCl-mediated modification of tyrosine and methionine, whereas tryptophan loss was enhanced. At higher NO concentrations, enhanced tyrosine and methionine loss was detected. These analytical data were confirmed in studies of cell adhesion and metabolic activity. Together, these data indicate that endogenous plasma levels of SCN (but not Br or I) can modulate protein modification induced by MPO, including the extent of chlorination. In contrast, NO alters the type of modification, but does not markedly decrease its extent, with chlorination replaced by nitration. These data also indicate that MPO could be a major source of nitration in vivo, and particularly at inflammatory sites where NO levels are often elevated.