Osteoarthritis is often accompanied by disturbance of oxidative equilibrium. The aim of the study was to analyze antioxidant defense system function in patients with hip and knee osteoarthritis by assessing superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) activity in erythrocytes. The study included 26 women and 18 men who underwent hip or knee arthroplasty. The pre- and postoperative activities of the antioxidant enzymes were evaluated. SOD activity was determined using the Misra/Fridovich method, CAT by the Beer/Seizer method, and the activity of GPx by the Paglia/Valentine method. The results were compared with those of a healthy control group. SOD activity of the patients was insignificantly lower than in the control group. However, after arthroplasty it increased in subgroups: in males by 13% (p<0.04), in females by 7% (p<0.05), in the group <69 years of age by ca. 14%) (p<0.01), and in the group with idiopathic OA by 12% (p<0.005). The activities of CAT and GPx in all the patient groups were significantly higher than in the controls. It increased even more on the 10th day after arthroplasty with the exception of the patients with rheumatoid osteoarthritis, in whom a decrease by 65.5% (p<0.008) were found, and in the patients > or =69 years old by 9.3% (NS). Knowledge of antioxidant enzyme activity might offer new targets for future therapeutic methods for the treatment of degenerative joint disease.