Peroxynitrite, formed by reaction of superoxide and nitric oxide, appears to be an important tissue-damaging species generated at sites of inflammation. In this paper, we compare the abilities of several biological antioxidants to protect against peroxynitrite-dependent inactivation ofα1-antiproteinase, and to inhibit tyrosine nitration upon addition of peroxynitrite. GSH and ascorbate protected efficiently in both systems. Uric acid inhibited tyrosine nitration but notα1-antiproteinase inactivation. The possibility that ascorbic acid is an important scavenger of reactive nitrogen speciesin vivois discussed.