Saponins with an aldehyde function bound at C-4 from different plant origins increase the cytotoxicity of the lectin agrostin by enhancing its penetration through the cell membrane. Experiments with different pure saponins in combination with agrostin showed that also the glycosidic part of acidic bisdesmosidic saponins especially the oligosaccharidic ester chain at C-28 plays an important role in the potentiation of agrostin's cytotoxicity as result of an interaction between saponins and lectins at the cell membrane.