There is a close relationship between inflammation and reactive oxygen species (ROS). Primary source of ROS are activated leucocytes. Antioxidant system protects organism against the deleterious effect of ROS. The aim of the present study was to follow the activity of antioxidant system in blood and colonic mucosa of 17 patients with idiopathic proctocolitis. All patients were treated with 5-aminosalicylic acid (Salofalk), 7 patients with a combination of prednison. The following biochemical parameters were ascertained: malondialdehyde (MDA) and ceruloplasmin in serum, glutathione (GSH) and activities of superoxide dismutase, catalase and glutathione peroxidase in erythrocytes and colonic mucosa. Before treatment there was found: an increase of MDA and all antioxidant enzyme activities, an a decrease of GSH. After 3 weeks of therapy in 59% of patients, initial clinical remission was observed without serious improvement of biochemical parameters. After 10 weeks of therapy the clinical course improved in all patients, a significant decrease of MDA and activities of all antioxidant enzymes as well as an increase of GSH were stated. The authors assume that IP was positively affected by 5-ASA, and in some patients by its combination with prednisom. The effect of 5-ASA is based on its known antiinflammatory impact and its functioning as a "scavanger" of free radicals (or ROS). (Tab. 1, Fig. 3, Ref. 23.)