A new 2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP)-conjugated saponin, lablab saponin I, was isolated from the hypocotyl of hyacinth bean (Dolichos lablab). The structure was elucidated by 1 H NMR and 1 3 C NMR spectroscopy and chemical techniques as 3-O-[α-l-rhamnopyranosyl-(1 → 2)-β-d-galactopyranosyl(1 → 2)-β-d-glucuronopyranosyl(1 → )]-22-O-[2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one(2 → )]-3β,22β,24-trihydroxyolean-12-en-28-al. SOD (superoxide dismutase)-like activity depended upon the DDMP group and the aldehyde group (C-28) of the aglycone was observed in the order of lablab saponin I > glutathione > soyasaponin βg > maltol. The soybean saponin Bb, lacking the DDMP moiety was found not to exhibit the SOD-like activity.