Five dibenzylbutyrolactones were isolated from a methanol extract of Arctii fructus (Arctium lappa L.) by bioassay-guided isolation, using the interleukin-l $eta$ converting enzyme (caspase-l, ICE) production inhibitory assay in vitro. These compounds were spectroscopically identified as lappaol E (1), lappaol A (2), matairesinol (3), arctigenin (4), and arctiin (5). Among the compounds tested, arctigenin (4) showed the strongest inhibitory activity for ICE production in IL-$eta$-induced proliferation of D 1 OS cells. Western blot analysis demonstrated that the arctigenin suppressed the expression of ICE protein in a dose-dependent manner. To estimate the biotransformation of Arctii fructus in vivo by human intestinal bacteria, we carried out an anaerobic incubation of the Arctii fructus extract with a human fecal suspension. From the HPLC analysis of metabolites, Arctiin (IC$_{50}$=74.2$mu$g/ml), a major component of Arctii fructus, was transformed to aglycone, arctigenin (IC$_{50}$=12.5$mu$g/ml), by human intestinal bacteria. The ICE production inhibitory activity of Arctii fructus would be much stronger in vivo than in vitro due to the biotransformation by human intestinal bacteria.