In this work we investigate the antioxidant properties of an aqueous extract prepared from an infusion of Ilex paraguariensis (Aquifoliaceae) using free radical-generating systems. The extract inhibited the enzymatic and nonenzymatic lipid peroxidation in rat liver microsomes in a concentration-dependent fashion, with IC50 values of 18 mu g/ml and 28 mu g/ml, respectively. The extract also inhibited the H2O2-induced peroxidation of red blood cell membranes with an IC50 of 100 mu g/ml and exhibited radical scavenging properties toward superoxide anion (IC50 = 15 mu g/ml) and 2,2-diphenyl-1-picrylhydrazyl radical. In the range of concentrations used, the extract was not a scavenger of the hydroxyl radical. Our results suggest that ingestion of extracts of Ilex paraguariensis could contribute to increase the antioxidant defense of an organism against free radicals attack.