Soybean saponin is one of several soybean glycosides. Recently, new soybean saponins which possess the DDMP (2,5-dihydroxy-6-methyl-2,3-dihydro-4H-pyran-4-one) moiety have been detected. Soyasaponin beta g of these new soybean saponins was shown to inhibit hydrogen peroxide damage to mouse fibroblast cells. Not only the new saponin but also soyasaponin I, which has lost the DDMP moiety from soyasaponin beta g, was tested and showed higher inhibition in this assay. In order to investigate the relationship between the effect and chemical structure, glycyrrhizin, which has a similar chemical structure to soyasaponin I, was also tested and exhibited the same effect as soyasaponins beta g and I. Soyasaponin Ab, a major authentic bisdesmosidic saponin that exists only in the soybean hypocotyl, was also tested and exhibited the highest inhibition of hydrogen peroxide damage among the four saponins. It was concluded that water-soluble soybean saponins protected the cells from damage by hydrogen peroxide.