The effect of pH and hemoglobin on oxidation of the microsomal lipids of cod was determined in isolated microsomes and in washed cod muscle. An increase of hemoglobin concentration from 0.5 to 15 microM accelerated lipid oxidation in both systems. In cod microsomes the rate of lipid oxidation increased in the order pH 6.8 >> pH 7.6 > pH 8.4 > pH 6.0 > pH 3.5. However, in washed cod muscle a decrease of pH from 7.8 to 6.8 greatly increased the lag phase and decreased the rate of lipid oxidation. A further decrease in pH to 3.5 decreased the lag phase and increased the rate of lipid oxidation further. A decrease of pH from 7.6 to 6.4 greatly reduced the affinity of hemoglobin for oxygen. Formation of methemoglobin due to autoxidation occurred more rapidly at pH 6.0 than at pH 7.5. Structural changes of the isolated microsomal membranes could be the reason for the unexpected slow lipid oxidation in microsomes at pH 6.0 and below.