Cells and tissues are sensitive to oxidative stress, caused by the formation of free radicals. If not deactivated by antioxidants, organic peroxides and hydroperoxides are the first reaction products between cellular constituents and free radicals or other reactive oxygen derivates. The determination of the oxidative status / oxidative stress is essential in today's medical research and diagnostics. Methods used so far were either expensive (HPLC), or detected only degradation products of polyunsaturated fatty acids, like TBARS (thiobarbituric acid reactive substances). The Biomedica OxyStat assay measures the total peroxide concentration of a sample, utilizing a quick and simple assay procedure. Results show a direct correlation between free radicals and circulating biological peroxides and thus allow the characterization of the oxidative status in biological samples.