When ginsenosides Rbl, Rb2, and Rc were anaerobically incubated with commercial and human intestinal lactic acid bacteria, most commercial lactic acid bacteria did not metabolize these ginsenosides to compound K. However, lactic acid bacteria, B. minimum KK-1, Bifidobacterium cholerium KK-2, and B. cuniculi K-513, isolated from human intestinal microflora transformed these ginsensosides to compound K. When the bacterial mixtures of commercial lactic acid bacteria were incubated with these ginsenosides, these compounds were not transfformed to compound K. However, when Bzfidobacterium KK-1 and KX-2 were miked, these ginsenosides were synergistically transformed to compound K. When water extract of ginseng was incubated with these mixed bifidobacteria, compound K was potently produced. Therefore, it is suggested that, if ginseng with these mixed bifidobacteria is fermented, compound K-enforced ginseng materials could be produced that show cytotoxicity against tumor cell lines.