An extract of the edible parts of Hosta sieboldiana suppressed the growth of P388 mouse leukemic cells. A new steroidal saponin was isolated from Hosta sieboldiana in this study, and the structure was determined by a spectroscopic analysis to be (25R)-3beta-(alpha-L-rhamnopyranosyl-(1-->4)-beta-D-glucopyranosyl-(1-->3)-[beta-D-glucopyranosyl-(1-->2)]-beta-D-glucopyranosyl-(1-->4)-beta-D-galactopyranosyl)-5alpha-spirostan-2alpha-ol.