Saikosaponins, the active principles of Bupleuri Radix, are generally sparingly soluble in water. The solubilizing effect of Ginseng saponins on saikosaponin-b₁ (Sb₁) which is formed from saikosaponin-a (Sa) by mild acid treatment, was investigated. It was revealed that the significant increase of the water-solubility of Sb₁ with Ginseng saponin mixture was due mainly to malonyl-ginsenosides. Some solubilizing effect was also observed with ginsenoside-Ro (Ro). No solubilizing effect was found with the neutral dammarane saponins, while the effects of malonyl-ginsenosides and Ro were remarkably potentiated in the presence of these neutral dammarane saponins. The water-solubility of Sa was also increased in the presence of malonyl-ginsenosides under cooperation with the neutral dammarane saponins.