In the course of our phytochemical studies of different plants from developing countries we isolated and structurally characterized several flavonoid derivatives (compounds 1–26), both aglycones and glycosides, typical of the species investigated. The aim of this study was to evaluate varieties of medicinal plants that were growing in developing countries, known in traditional medicine as anti-inflammatory remedies, with respect to their flavonoidic isolated constituents, assuming that their anti-inflammatory activity could be explained, at least in part, by the presence of antioxidant principles. The antioxidant activities of compounds 1–26 were evaluated by measuring their ability to scavenge the radical cation 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS+) and the superoxide anion, to inhibit β-carotene oxidation in a lipid micelle system, and to inhibit xanthine oxidase activity, showing some structure–activity relationships.