A North American ginseng extract (NAGE) containing known principle ginsenosides for Panax quinquefolius was assayed for metal chelation, affinity to scavenge DPPH-stable free radical, and peroxyl (LOO*) and hydroxyl (*OH) free radicals for the purpose of characterizing mechanisms of antioxidant activity. Dissociation constants (Kd) for NAGE to bind transition metals were in the order of Fe2+ > Cu2+ > Fe3+ and corresponded to the affinity to inhibit metal induced lipid peroxidation. In a metal-free linoleic acid emulsion, NAGE exhibited a significant (p < or = 0.05) concentration (0.01-10 mg/mL) dependent mitigation of lipid oxidation as assessed by the ammonium thiocyanate method. Similar results were obtained when NAGE was incubated in a methyl linoleate emulsion containing haemoglobin catalyst and assessed by an oxygen electrode. NAGE also showed strong DPPH radical scavenging activity up to a concentration of 1.6 mg/mL (r2 = 0.996). Similar results were obtained for scavenging of both site-specific and non site-specific *OH, using the deoxyribose assay method. Moreover, NAGE effectively inhibited the non site-specific DNA strand breakage caused by Fenton agents, and suppressed the Fenton induced oxidation of a 66 Kd soluble protein obtained from mouse brain over a concentration range of 2-40 mg/mL. These results indicate that NAGE exhibits effective antioxidant activity in both lipid and aqueous mediums by both chelation of metal ions and scavenging of free radicals.