Etiology and pathogenesis of endemic nephropathy (EN) are unknown and in this work the possible pathogenetic role of disturbed antioxidant protection, estimated by the activity of erythrocyte catalase, was evaluated. In patients with EN as well as in clinically healthy members of their families a statistically significant increase in catalase activity (16.7 +/- 0.63 x 10(4) U/g Hb) compared to the control group of the blood bank donors from the city of Nish (11.7 +/- 0.69 x 10 U/g Hb) was found. Increased catalase activity, at the level of significance, was also found in clinically healthy members of EN patients families. The increase of catalase activity was considered compensatory to the increased oxydative capacity.