The effects of diets differing in their level of unsaturation and their contents of natural antioxidants and prooxidants on lipid oxidation was measured in microsomal membranes of the thigh muscles of broilers by three processes: enzymatic, nonenzymatic and sarcoplasmic protein oxidation. Lysosomal cysteine proteinase activities were determined to assess the influence of the different diets on the enzymatic proteolysis of meat. Statistically significant differences were found in the enzymatic reaction for 72 h of incubation at 4°C in relation to the effect of both diet and antioxidant supplementation. Meat from animals fed an unsaturated diet containing sunflower oil gave the highest level of oxidation. Broilers fed supplemental antioxidants, especially α-tocopherol, had lower degrees of oxidation than those fed the basal diet. An unexpected result of the prooxidant diet study was a higher degree of oxidation in samples from animals deprived of iron and copper. Cysteine proteinase activities were favoured by vitamin E (α-tocopherol) supplementation and the absence of copper. ©