An antioxidant was purified from human placenta extract (HPE) by using gel filtration, liquid-liquid extraction, silicagel column chromatography, and HPLC. The purified antioxidant was identified to be L-tryptophan (L-Trp). L-Trp showed higher inhibitory activity than mannitol and DMSO on the Fenton reaction-induced degradation of 2-deoxy-D-ribose. L-Trp also had much higher inhibitory activity on the cytochrome P-450-dependent lipid peroxidation than the previously identified antioxidants of HPE, L-phenylalanine, L-tyrosine and uracil. On the other hand, the inhibitory effect of L-Trp on the Fenton reaction-induced protein oxidation was smaller than that of uracil. These results suggest that L-Trp is a main antioxidant of HPE of which the effect is based on the suppression of lipid peroxidation in the oxidative stress status.