The singlet oxygen quenching activities of carotenoids, beta-carotene, free astaxanthin, its monoester and its diester, were examined in vitro by a simple and rapid method for the measurement of Methylene Blue-sensitized photooxidation of linoleic acid in the hexane/ethanol solvent system. The concentrations of carotenoids, beta-carotene, free astaxanthin, its mono- and di-ester, required for 50% inhibition of lipid oxidation were 40, 8, 9, and 0 mu M in 100% ethanol and, 14, 16, 10, and 7 mu M in 50% (v/v) hexane in ethanol, respectively. Astaxanthin esters function as powerful antioxidant agents under both hydrophobic and hydrophilic conditions.