A new fluorometric determination of hydroperoxides was proposed. Non-fluorescent diphenyl-1-pyrenyl-phosphine (DPPP) was oxidized quantitatively by hydroperoxides to a strong fluorescent DPPP oxide. A sensitive assay for lipid hydroperoxides was developed based on the reaction. The reaction of lipid hydroperoxides with DPPP was conducted in the dark in the mixture of chloroform and methanol at 60°C within 60 min. The contents of hydroperoxides showed linear relation to the fluorescence intensities in a wide concentration range. The sensitivity was 10,000 times higher than conventional iodometry. The correlation constant between peroxide values obtained by iodometry and by this method was 0.9993 (n=10). The sample size was minimized to less than 3 mg per tube.