Peroxynitrite (ONOO-) is a 'reactive nitrogen species' that can be formed (among other reactions) by combination of superoxide (O2.-) and nitric oxide (NO.) radicals. It is being increasingly proposed as a contributor to tissue injury in several human diseases. The evidence presented for peroxynitrite participation usually includes the demonstration of increased nitrotyrosine levels in the injured tissue. Indeed, this is often the only evidence presented: the assumption is that formation of nitrotyrosine is a biomarker specifically diagnostic of ONOO- production. The present article examines this assumption and concludes that nitrotyrosine is a biomarker for 'nitrating species' rather than being specific for ONOO-.