Supeoxide dismutase, glutathione peroxidase, glutathione S-transferase, and the p-phenylenediamine oxidase activity of ceruloplasmin, as well as lipid peroxides in terms of malondialdehyde, were determined in the blood of 52 breast cancer patients and 29 healthy women. No differences except for superoxide dismutase were found regarding lipid peroxide level and antioxidant enzyme activities in blood plasma and red blood cells between breast cancer patients and matched control women. Superoxide dismutase activity was statistically significantly lower in breast cancer patients than in the control group (p<0.0002). We also found a statistically significant correlation between the p-phenylenediamine oxidase activity of ceruloplasmin and lipid peroxide concentration in the blood plasma of the patients with breast cancer (r=0.342, p<0.05).