Generation of reactive oxygen species in vivo results in oxidative-damage to cellular components, including proteins. Due to the relatively long half-lives of several blood proteins the cumulative formation of oxidatively damaged proteins might serve as a biomarker for reactive oxygen species formation. The most prominent sources of reactive oxygen species in vivo are site-specific metal ion-catalyzed reactions of the Fenton and Haber-Weiss types and the H2O2/peroxidase system. In vitro oxidation of L-tyrosine using a peroxidase or Cu++/H2O2 system gives rise to the formation of a highly fluorescent substance, bityrosine. High-performance liquid chromatography (HPLC) analysis of acid hydrolyzed serum albumin after oxidation with peroxidase/H2O2 or with Cu++/H2O2 showed that bityrosine had been formed whereas oxidation of this protein with Fe(III)/ascorbate did not result in the formation of bityrosine. Bityrosine could not be detected in human plasma proteins or haemoglobin with the detection limit of one pmol per mg protein.