Peroxynitrite may be formed in the vasculature by the reaction of puperoxide with nitric oxide. When the blue copper-containing protein, caeruloplasmin, is incubated with peroxynitrite, copper is released, and ferroxidase activity and the blue colouration are lost. When plasma from normal subjects is incubated with peroxynitrite, the oxidant reacts with numerous plasma constituents but is still able to release copper from caeruloplasmin. As the ferroxidase activity of caeruloplasmin is lost in plasma in the presence of peroxynitrite, a second ferroxidase activity associated with peroxidised lipids, and not inhibited by azide, is formed. © 1994.