Cupric ions (Cu²⁺) and ferric ions (Fe³⁺) added to hydrogen peroxide generate hydroxyl radicals (OH^. capable of degrading deoxyribose with the formation of thiobarbituric acid-reactive products. This damage can be inhibited by catalase, OH^. radical scavengers and specific metal ion chelators. All proteins tested nonspecifically inhibited copper-dependent damage but have little effect on the iron-dependent reaction. Copper ions appear to bind to the proteins which prevents formation of OH^. radicals in free solution. However, OH^. radicals are still generated at a site-specific location on the protein molecule. Protein damage is detected as fluorescent changes in amino acid residues.