The effects of sulphasalazine, 5-aminosalicylic acid (5-ASA), and sulphapyridine on peroxidation of red cell membrane lipids, measured as malondialdehyde production, were assessed. Sulphasalazine and 5-ASA, at concentrations of 10(-5)-10(-3) M significantly inhibit lipid peroxidation, suggesting an antioxidant action that may explain the efficacy of these drugs in treating inflammatory bowel disease. Sulphapyridine, which is not effective in inflammatory bowel disease inhibited malondialdehyde production at a concentration of 10(-3) M only.