Biomolecules can be oxidized by free radicals. This oxidative damage has an important etiological role in aging and the development of diseases like cancer, atherosclerosis, and other inflammatory disorders. Synthetic antioxidants, like butylated hydroxyanisole, are good free radical scavengers, however, the synthetic antioxidants can be carcinogenic. Therefore, there is an increasing interest in searching for antioxidants of natural origin. We report here the results of a screening for antioxidant activity of 53 ethanolic extracts from 40 Peruvian plants used in traditional medicine for the treatment of several infectious and inflammatory diseases. The antioxidant activity in vitro was measured by means of the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging assay. Rutin, a natural antioxidant, was used as a control. Twenty one (39.6%) of the plant extracts showed an EC50 lower than 50 μg/ml. The plants with the highest activity were Gentianella nitida, Iryanthera lancifolia, Lepechinia meyenii, Oenothera multicaulis, Philodendron solimoesense and Tetracera volubilis, which showed an EC50 of 13.70, 14.08, 16.65, 16.89, 8.80, and 5.29 μg/ml, respectively. The crude ethanolic extract of Tetracera volubilis has better antioxidant activity in vitro than the pure natural antioxidant rutin (EC50 = 7.16 μg/ml). © ISHS 2005.