During the course of a study of leguminous plants, cytotoxicity was demonstrated by the crude saponin fraction of Albizia julibrissin. Following chromatographic purification, the structures of three novel saponins, julibrosides I-III (1-3), inclusive of a cytotoxic principle, were elucidated. A comparison of the cytotoxicity of julibrosides (1-3) and their prosapogenins (4-15) prepared by alkaline hydrolysis clearly indicated that both an alpha-L-arabinofuranosyl-(1-->4)-[beta-D-glucopyranosyl-(1-->3)]-alpha- L-rhamnopyranosyl-(1-->2)-beta-D-glucopyranosyl ester unit and a monoterpene-quinovopyranosyl moiety are crucial substituents for cytotoxicity among this class of compounds. The hydroxy group at C-16 of aglycon may play an important role in mediating cytotoxicity, and the N-acetyl-glucosamine moiety at C-3 seems to enhance activity because 3 showed the strongest cytotoxicity.