Polyphenolic flavonoids are powerful antioxidants. In the present study we investigated the antioxidative activity against low-density-lipoprotein (LDL) oxidation of a not yet studied subclass of polyphenols, the isoflavans, which are present in licorice alcoholic extract. The study was performed in humans as well as in atherosclerotic apolipoprotein E-deficient mice (E zero), because their LDL is highly susceptible to oxidation. LDL oxidation was induced by incubating it with copper ions as well as with the aqueous or lipid-soluble free radical generators 2,2'-azobis'2-amidino propane hydrochloride (AAPH) and 2,2'-azobis 2,4-dimethylvaleronitrile (AMVN), respectively. The extent of LDL oxidation was determined by measuring the formation of conjugated dienes, thiobarbituric acid reactive-substances (TBARS), and lipid peroxides. By all methods in human studies, licorice ethanolic extract as well as a pure material, which was identified by gas chromatography-mass spectroscopy as the isoflavan glabridin, were shown to inhibit LDL oxidation by a mechanism involving scavenging of free radicals. In an ex vivo study, LDL isolated from the plasma of 10 normolipidemic subjects who were orally supplemented for 2 wk with 100 mg licorice/d was more resistant to oxidation than was LDL isolated before licorice supplementation. Dietary supplementation of each E zero mouse with licorice (200 micrograms/d) or pure glabridin (20 micrograms/d) for 6 wk resulted in a substantial reduction in the susceptibility of their LDL to oxidation along with a reduction in the atherosclerotic lesion area. These results could be related to the absorption and binding of glabridin to the LDL particle and subsequent protection of the LDL from oxidation by multiple modes as shown in humans and in E zero mice.