The effects of soybean saponins (SS) and gypsophilla saponin (GS) on the growth and viability of colon tumor (HCT-15) cells in culture were investigated. Cells were incubated in various concentrations of saponins for 1 hour (short term) or 48 hours (long term). Cell growth and viability were monitored at 24 and 48 hours. SS and GS inhibited cell growth and reduced cell viability in a dose-dependent manner in long-term treatment. The viability of cells was also reduced by short-term treatment with GS. The saponins differed in their effects on cell surface morphology: GS induced a rough and granular cell surface, whereas SS-treated cells displayed only minor morphological alterations. Changes in membrane permeability were assessed by measuring leakage of the cytoplasmic enzyme lactate dehydrogenase from cells. GS showed a concentration-dependent increase in lactate dehydrogenase leakage, whereas SS did not exhibit this effect. These results suggest that SS and GS have a significant growth-inhibitory effect on colon tumor cells in culture. However, it would appear that they are acting through different mechanisms.