It has been investigated whether secoisolariciresinol (SECO) and anhydrosecoisolariciresinol (AHS), an acid degradation product of SECO, could be fermented in a similar way, and to a similar extent, by members of the intestinal microbiota. AHS and SECO were demethylated by Peptostreptococcus productus, Eubacterium limosum and Clostridium methoxybenzovorans. These bacteria have been identified as members of the human intestinal flora or closely related species. Demethylated AHS and demethylated SECO were purified by preparative RP-HPLC, and subsequently subjected to fermentation with Eggerthella lenta, Clostridium scindens and Clostridium hiranonis. Eggerthella lenta efficiently dehydroxylated demethylated SECO to enterodiol, whereas the other bacteria showed no dehydroxylation activity. The conversion of the diol structure of SECO into the furan ring in AHS did not influence the demethylation capability of the tested bacteria. The results also showed that the extent of dehydroxylation of demethylated AHS was much lower than that of demethylated SECO. Plant lignans are converted into bioactive mammalian lignans by the human intestinal bacteria. This study showed that the modification of plant lignans resulted in the formation a new type of mammalian lignan.