Objective. The present study was undertaken to evaluate the activities of some key erythrocyte and plasma enzymes participating in free radical metabolism and the end product of lipid peroxidation in rheumatoid arthritis, and whether there are any differences for these parameters between newly diagnosed untreated patients and rheumatoid arthritis patients on drug therapy. Patients and methods. Superoxide dismutase and catalase activities, and malondialdehyde levels were determined in erythrocytes and plasma samples from 54 patients with rheumatoid arthritis (21 of whom without any treatment and 33 on classical therapy regimens) and from 33 healthy controls. Results. There were no statistically significant differences in mean values of activities of the erythrocyte enzymes between the patients and controls. Malondialdehyde levels were significantly increased in both newly diagnosed untreated patients and patients on drug therapy compared to control subjects. Malondialdehyde levels were lower in the treated group than the newly diagnosed untreated group (0.214 +/- 0.111 mu mol/L and 0.388 +/- 0.075 mu mol/L, respectively) (P < 0.0001). Mean plasma superoxide dismutase activity was lower in the group of newly diagnosed untreated patients compared to those of the treated and control groups (1.31 +/- 0.69 U/mL, 1.79 +/- 0.94 U/mL and 2.48 +/- 0.95 U/mL, respectively) (P < 0.0001, untreated vs control groups). Conclusions. These results suggest sufficient antioxidant enzyme activities in erythrocytes in patients with rheumatoid arthritis and also increased lipid peroxidation end products in newly diagnosed untreated patients compared to control group and patients on drug therapy. (C) 2001 Editions scientifiques et medicales Elsevier SAS.