We have analysed products of lipid peroxidation reactions and activities of antioxidant enzymes in cancerous breast tissue and in corresponding reference tissue. In addition, the serum lipid peroxidation and peroxyl-radical-trapping capacity of breast cancer patients were compared to those of healthy subjects. A total of 23 patients with breast cancer participated in this study. In the cancerous tissue, catalase activity was lower than in the reference tissue, while the activities of superoxide dismutase, glutathione peroxidase and the hexose monophosphate shunt were elevated. The content of thiobarbituric-acid-reactive material was slightly lower in the cancerous tissues, but the levels in serum were found to be elevated in patients with breast cancer. The amounts of conjugated diene double bonds were essentially equal both in the cancerous and in the reference tissue. Moreover, in breast cancer patients the serum levels of diene conjugation and the peroxyl-radical-scavenging capacity did not differ from those measured in healthy subjects. This study indicates that the antioxidant defence system is altered in cancerous breast tissues, but does not support the hypothesis suggesting that formation of lipid peroxides in the tumour tissue itself is of primary importance in the carcinogenesis.