A spectrophotometric method is described for the measurement of lipid hydroperoxides, to a lower limit of 0.5 nmoles, based on the formation of triiodide ions measured at the absorbance maximum of 365 nm. The assay mixture, which was modified from an earlier published procedure [El-Saadani, M., Esterbauer, H., El-Sayed, M., Goher, M., Nassar, A.Y., and Jürgens, G. (1989) J. Lipid Res. 30, 627-630], contains 18% methanol together with nonionic and cationic detergents, and is designed so that the hydroperoxides to be measured can be added in either water or methanol. By incubating the reaction mixture at 50 degrees C for 30 min, less-reactive hydroperoxides can be measured with the same fidelity as more-reactive ones. Under these conditions, the assay can be carried out under ordinary room lighting and without special protection from ambient oxygen with absorbance values being stable up to 15 h. Enzymatic standardizations showed that the triiodide method gave comparable results for H2O2, cumene hydroperoxide, linoleic acid hydroperoxide, phosphatidylcholine hydroperoxide, and a photooxidized tissue extract containing a mixture of hydroperoxides. The triiodide assay is recommended primarily for measuring purified hydroperoxides.