Ferritin was extracted from bovine diaphragm muscle to determine whether in situ ferritin concentrations could catalyze oxidation of phosphatidylcholine liposomes. Production of thiobarbituric acid-reactive substances by the ferritin-containing fraction increased with increasing ascorbate and cysteine (10–500 μ.M). Ascorbate stimulated more lipid oxidation than cysteine. Catalysis of lipid oxidation by ascorbate and the ferritin-containing fraction increased slightly with increasing pH from 5.0 to 7.0. Ferritin/ascorbate was inhibited by superoxide dismutase, EDTA and ceruloplasmin and stimulated by hydrogen peroxide. Data suggest ferritin may be partially responsible for catalysis of lipid oxidation in muscle foods.