When ginseng water extract was incubated at $60^{circ}C$ in acidic conditions, its protopanaxadiol ginsenosides were transformed to ginsenoside $Rg_3$ and ${Delta}^{20}$-ginsenoside $Rg_3$. However, protopanaxadiol glycoside ginsenosides $Rb_1, Rb_2$ and Rc isolated from ginseng were mostly not transformed to ginsenoside $Rg_3$ by the incubation in neutral condition. The transformation of these ginsenosides to ginsenoside $Rg_3$ and ${Delta}^{20}$-ginsenoside $Rg_3$ was increased by increasing incubation temperature and time in acidic condition: the optimal incubation time and temperature for this transformation was 5 h and $60^{circ}C$ resepectively. The transformed ginsenoside $Rg_3$ and ${Delta}^{20}$-ginsenoside $Rg_3$ were metabolized to ginsenoside $Rh_2$ and $Delta^{20}$--ginsenoside $Rh_2$, respectively, by human fecal microflora. Among the bacteria isolated from human fecal microflora, Bacteroides sp., and Bifidobacterium sp. and Fusobacterium sp. potently transformed ginsenoside $Rg_3$ to ginsenoside $Rh_2$. Acid-treated ginseng (AG) extract, fermented AG extract, ginsenoside $Rh_2$ and protopanaxadiol showed potent cytotoxicity against tumor cell lines. AG extract, fermented AG extract and protopanaxadiol potently inhibited the growth of Helicobacter pylori.