In the course of our experiments on the metabolic conversion of lignans to the estrogenic substances enterodiol (END) and enterolactone (ENL) by human intestinal flora, we isolated two anaerobes, Ruminococcus sp. END-1 and strain END-2, capable of oxidizing END. The former selectively converted (−)-END to (−)-ENL, while the latter selectively converted (+)-END to (+)-ENL, indicating enantioselective oxidation by intestinal bacteria.