Developed as a membrane probe by Hudson and co-workers, the naturally occurring polyunsaturated fatty acid cis-parinaric acid is the closest structural analog of intrinsic membrane lipids among currently available fluorescent probes. Because oxidation of the probe is accompanied by decreased fluorescence and absorption, cis-parinaric acid has been widely used in lipid peroxidation assays, including evaluation of antioxidants. Our cis-parinaric acid is packaged as a 3 mM solution in deoxygenated ethanol containing 10 µg/mL of butylated hydroxytoluene (BHT).