The antioxidative activities of the plant lignans secoisolariciresinol and matairesinol and the mammalian lignans enterodiol and enterolactone were investigated using the FRAP assay (Ferric Reducing/Antioxidant Power assay). The plant lignans showed a clearly higher antioxidant activity than the known antioxidant ascorbic acid. The mammalian lignans were far less active in this assay. Using 2-methoxy-p-cresol and m-cresol as structural model compounds for plant and mammalian lignans, the additional aromatic methoxy group was shown to be responsible for the different antioxidant activities. (C) 2002 Elsevier Science Ltd. All rights reserved.