Abstract Dihydroxyfumaric acid induces lipid peroxidation in rat liver mitochondria as reported previously. When the mitochondria were solubilized with 0.35% ( W V ) sodium cholate, the supernatant itself could not catalyze lipid peroxidation with dihydroxyfumaric acid, but the precipitate slightly induced the reaction. The supernatant produced lipid peroxide in the presence of the precipitate and dihydroxyfumaric acid. The supernatant was heat sensitive contrary to the stability of the precipitate. An attempt was made to isolate active entity through a sephadex G-200 column and a DEAE-cellulose column, resulting in about 10-fold purification. At 408–410 nm the partially purified agent showed a maximum absorption, which disappeared rapidly after reduction with sodium dithionite and was slowly diminished with dihydroxyfumaric acid. The molecular weight was much larger than that of oxidized cytochrome c.