ABSTRACTA microsomal fraction which catalyzes the oxidation of the membrane lipid in the presence of NADH and Fe has been isolated from the muscle of red hake. Unlike other microsomal membrane systems previously reported which prefer, and in some cases require, NADPH, very little lipid oxidation is observed with NADPH in the fish muscle system. The enzymic nature of the reaction is indicated by the preference of NADH versus NADPH, the inhibitory effect of high concentrations of NADH, and the loss of the activity of the system upon heating. The effect of temperature on production of TBA‐reactive substances and the loss of NADH from the reaction medium was followed. There is a sharp decline in production of TBA‐reactive substances between 6°C and 0°C. Phosphate buffer was shown to inhibit the oxidation compared to histidine buffer at low concentrations of ADP. High concentrations of ADP were inhibitory to the lipid peroxidation system when histidine was used as b