Samples containing between 1 and 50 nmol of hydroperoxides in oxygen-free methanol-acetic acid containing I- were incubated at 50°C. This ensured reduction of all hydroperoxides tested in 15 min. Addition of sufficient Cd acetate to combine with the remaining I- allowed reading of absorbance of the I3- produced in open cuvettes. The absorbance was stable for several hours. It had a maximum at 358 nm with molar extinction coefficient of 2.97 × 104 1 mol-1 cm-1. Sensitivity of the assay could be improved by injecting the I3- into a C-18 HPLC column eluted with methanol-water-acetic acid solution. Both methods are potentially suitable for assay of hydroperoxides in a wide range of biological materials.