Linoleate oxidations by extracts of mackerel liver and by hemin were observed by manometric methods. The oxidative activity of the liver extracts was lost on boiling, but hemin was not influenced by heat. Although ordinary antioxidants inhibited linoleate oxidation due to liver extracts or hemin, there were certain differences in the inhibitory actions of other reagents (cyanide, fluoride, mercury, etc.) on the oxidative activity of liver extracts and hemin. The Fe content of liver extracts was considerably different from that of hemin. Fractioation with ammonium sulphate was tried, but Fe was not freed. Two fractions (above and below 0.7 saturation) showed almost a similar absorption of oxygen with linoleate substrate, but one was labile to heat and the other was not.