When saponin extracts of dried ginseng and red ginseng were anaerobically incubated with human intestinal microflora, these extracts were metabolized to compound K and ginsenoside $Rh_2$, respectively. However, when these extracts were incubated with commercial lactic acid bacteria, these did not metabolize these ginsenosides to compound K or ginsenoside $Rh_2$. Among some intestinal bacteria isolated from human feces, Bacteroides C-35 and C-36 transformed these saponin extracts to compound K and ginsenoside $Rh_2$, respectively. These bacteria also transformed water extracts of dried ginseng and red ginseng to compound K and ginsenoside $Rh_2$, respectively, similarly with that of the saponin extracts. Among transformed ginsenosides, compound K and 20(S)-ginsenoside $Rh_2$ exhibited the most potent cyotoxicity against tumor cells.