Seven sesquiterpene lactones isolated from Helenium aromaticum: helenalin, mexicanin I, linifolin A, geigerinin, and from Telekia speciosa: 6 alpha-hydroxy-2,3-dihydroaromaticin, asperilin, telekin have been tested for their hydroxyl radical scavenging activity and effect on lipid peroxidation. All compounds were found to be potent hydroxyl radical scavengers but did not affect lipid peroxidation in vitro. In vivo they exerted pro-oxidative properties and caused glutathione level depletion and elevation in glutathione peroxidase activity.