A study has been made of the factors that contribute to the decreased rates of lipid peroxidation under different pro-oxidant conditions in intact Novikoff tumour cells, and in microsomal suspensions prepared from Novikoff tumour cells, compared with isolated normal rat hepatocytes and microsomal suspensions prepared from normal rat liver. The pro-oxidant conditions were the addition of either NADPH, NADPH + ADP + iron, NADPH + CCl4 or ascorbate + iron to the experimental systems used, or exposure to γ-radiation. Contributory factors to the lower rates of lipid peroxidation observed include: (a) a significant decrease in the polyunsaturated fatty acid content of Novikoff cells or Novikoff microsomes; the decreases are especially marked for the C(20:4) and C(22:6) fatty acids; (b) a very marked reduction in NADPH-cytochrome c reductase; and (c) no detectable content of cytochrome P-450. Another, and in our opinion critical, contribution to the diminished rate of lipid peroxidation in the tumour material is the substantial increase in α-tocopherol relative both to total lipid and to methylene-interrupted double bonds in fatty acids. Moreover, the α-tocopherol is the major contributor to lipid-soluble chain-breaking antioxidant in lipid extracts of normal liver and of Novikoff tumour material.