The antioxidant and scavenging properties of kolaviron, a flavonoid extract of Garcinia kola seeds, were evaluated in a series of in vitro assays involving free radicals and reactive oxygen species. Kolaviron exhibited markedly reducing power and antioxidant activity by inhibiting the peroxidation of linoleic acid. Kolaviron exhibited 57% scavenging effect on superoxide at a concentration of 1 mg/ml and 85% scavenging effect on hydrogen peroxide at a concentration of 1.5 mg/ml. Similarly, kolaviron, at a concentration of 2 mg/ml, elicited an 89% scavenging effect on a,a-diphenylb-picrylhydrazyl radical, indicating that the extract has effective activities as a hydrogen donor and as a primary antioxidant to react with lipid radicals. Kolaviron reacted with hydroxyl radical ( . OH) by inhibiting deoxyribose oxidation induced by a Fenton-type reaction system (Fe 3+ + EDTA/H 2 O 2 /ascorbic acid). The second-order rate constant for the reaction with . OH was approximately 1.1 ¥ 10 10 M -1 sec -1 . Kolaviron was effective at preventing microsomal lipid peroxidation induced by iron/ascorbate in a concentration dependent manner. The overall antioxidant activity of kolaviron on lipid peroxidation might be attributed to its properties of scavenging free radicals and active oxygen species and may relate directly to prevention of propagation of in vivo lipid peroxidation.