When ginsenoside R(c) was anaerobically incubated with human fecal microflora, all specimens metabolized ginsenoside R(c) to compound K and protopanaxadiol. The main metabolite was compound K. Among the bacteria isolated from human fecal microflora, most bacteria, such as Bacteroides sp., Eubacterium sp., and Bifidobacterium sp. potently transformed ginsenoside R(c) to compound K. Bifidobacterium K-103 and Eubacterium A-44 transformed it to compound K via ginsenoside R(d) and Bacteroides HJ-15 and Bifidobacterium K-506 metabolized to compound K via ginsenoside Mb, which was isolated as a new metabolite (M.W. 940[+Na]). Among ginsenoside R(c) and its metabolites, compound K exhibited the most potent antiallergic activity on the IgE-induced RBL cell line as well as potent cytotoxic activity against tumor cell lines.