Peptostreptococcus intermedius, one of the dominant bacteria of human intestine, reduced sennidin and sennoside to rheinanthrone and 8-glucosyl-rheinanthrone, respectively, and these reduction rates were stimulated by the addition of nicotinamide adenine dinucleotide (reduced form) (NADH), flavin adenine dinucleotide (FAD) and glucose. The reduction was accelerated by removing oxygen from the incubation tubes, which indicates the inhibition of the reduction by O2. Thus, for the maximal activity, the reaction system required an anaerobic condition and cofactors, NADH and FAD. This bacterium also reduced methyl orange, whose mode of reduction resembled that of sennidin reduction. More than 80% of these activities were recovered in the supernatant fraction after sonication of the bacterial suspension, and these activities were purified together by means of Sephadex G-200 gel-filtration and diethylaminoethyl-cellulose column chromatography. The purified enzyme reduced FAD and benzyl viologen in the presence of NADH under an anaerobic condition.