Cystatins are thiol proteinase inhibitors ubiquitously present in mammalian body. In brain, they prevent unwanted proteolysis and are involved in several neurodegenerative diseases. In the present study, it has been demonstrated that photoactivated riboflavin and H2O2 induced modifications of high molecular mass goat brain cystatin (HM-GBC) leads to its inactivation and degradation. It was found that the damage with both the oxidants occurred mainly because of the hydroxyl radicals. It has been also proposed that susceptibility of HM-GBC to oxidation by reactive oxygen species generated in vivo arise from oxidative modifications may lead to damage of this significant protein as it is so well pronounced, in vitro.