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Six bisdesmosidic bayogenin saponins, named tacacosides A1, A2, B1, B2, B3 and C, were isolated from the fruit and aerial parts of Sechium pittieri (COGN.) C. Jeffrey and S. talamancense (WUNDERLIN) C. Jeffrey, Costa Rican cucurbitaceae plants. Their structures were elucidated on spectral and chemical evidence as follows. Tacacoside A1: 3-O[beta-D-glucopyranosyl-(1-->3)-beta-D-glucopyranosyl]bayogenin 28-O-(alpha-L-rhamnopyranosyl- (1-->3)-beta-D-xylopyranosyl-(1-->4)-beta-D-apiofuranosyl-(1-->3)] - alpha-L-rhamnopyranosyl-(1-->2)-alpha-L-arabinopyranosyl) ester, tacacoside A2: 3-O-[beta-D-glucopyranosyl-(1-->3)-beta-D-glucopyranosyl]bayogenin 28-O-(alpha-L-rhamnopyranosyl- (1-->3)-beta-D-xylopyranosyl-(1-->4)-[beta-D-xylopyranosyl-(1-->3)]-a lph a-L- rhamnopyranosyl-(1-->2)-alpha-L-arabinopyranosyl) ester, tacacoside B1: 3-O-[beta-D-glucopyranosyl]bayogenin 28-O-(alpha-L-rhamnopyranosyl-(1-->3)-beta-D-xylopyranosyl- (1-->4)-[beta-D-apiofuranosyl-(1-->3)]-alpha-L-rhamnopyranosyl-(1- ->2)-alpha- arabinopyranosyl) ester, tacacoside B2: 3-O-[beta-D-glucopyranosyl]bayogenin 28-O-(alpha-L-rhamnopyranosyl-(1-->3)- beta-D-xylpyranosyl-(1-->4)-[beta-D-xylopyranosyl- (1-->3)]-alpha-L-rhamnopyranosyl-(1-->2)-alpha-L-arabinopyranosyl) ester, tacacoside B3: 3-O-[beta-D-glucopyranosyl-(1-->3)-beta-D-glucopyranosyl] bayogenin 28-O-[alpha-L-rhamnopyranosyl-(1-->3)-beta-D-xylopyranosyl- (1-->4)-alpha-L-rhamnopyranosyl- (1-->2)-alpha-L-arabinopyranosyl] ester, and tacacoside C: 3-O-[beta-D-glucopyranosyl]bayogenin 28-O-[alpha-L-rhamnopyranosyl- (1-->3)-beta-D-xylopyranosyl-(1-->4)-alpha-L-rhamnopyranosyl-(1-->2)- alpha-L-arabinopyranosyl] ester. These saponins showed moderate antiproliferative activity (ED50:10-74 micrograms/ml) against MK-1, HeLa and B16F10 cells.