Abstract1.Rats' liver‐extract contains substances titratable with iodine (probably ascorbic acid and glutathione), which disappear after shaking for two hours at 37°. Erythrocytes contain a reducing system, capable of preventing this oxidation.2.The mechanism of the respiratory reaction nevertheless cannot be identical with that of the ascorbic acid system of Von Szent‐Györgyi.3.A reversible or irreversible process of blood destruction cannot be the cause of the Michaelis‐Salomon effect, for liver‐extracts cause only a slight oxidation of haemochromogen, and spleen extract does not augment the oxygen consumption of a combination of liver‐extract and e