Membranes isolated from tumor cells present profound alterations in their composition, structural organization, and functional properties. In this study we have reported some of these alterations in microsomal and plasma membranes of hepatomas with different growth rate and degree of differentiation. The chemical parameters studied were the phospholipid-to-protein, the cholesterol-to-protein, and the cholesterol-to-phospholipid ratios and the fatty acid composition of the phospholipids. The physical parameters were the molecular order (static) and the fluidity (dynamic), determined, respectively, as the order parameter [P2] and the correlation time tau R of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH). The functional property investigated was the ability of the membranes to undergo superoxide-induced lipid peroxidation, determined as byproduct (malondialdehyde and lipid hydroperoxides) formation and as changes in the fatty acid acyl residues. Changes in the physical state of the membrane, induced by oxy radicals, were also monitored during lipid peroxidation. A study of the antioxidant activity of the tumor cell, in terms of oxy radical enzymatic defenses (superoxide dismutase, glutathione peroxidase and catalase) was also performed. The main results obtained are the following: hepatoma membranes possess a lower phospholipid content and a lower degree of fatty acid unsaturation; on the other hand, the cholesterol-to-phospholipid ratio is increased; the physical state appears characterized by an increased rigidity (increased molecular order of the lipids and decreased fluidity); the membrane peroxidizability is markedly depressed and its order parameter, in contrast to liver membranes, does not increase with exposure to the action of O2- radicals; and the oxy radical enzymatic defense mechanisms are decreased. All these alterations increase with increasing growth rate and dedifferentiation of the tumor. Considering all of the data, we are inclined to think that tumor membranes are altered structurally and functionally in part as the result of an oxy radical-induced damage that takes place in vivo under conditions of increased oxygen toxicity.