In this paper, the steroidal saponin was isolated from natural Chinese drugs of yarn and allum macrostemon bunge. The promotive and fungistatic actions of steroidal saponin was studied. The power time curves of growth of Escherichia coli at different concentrations of steroidal saponin were determined by using a 2277 Thermal Activity Monitor. The rate constant of promotive and fungistatic actions was calculated. The relationship between growth rate constant with concentration was established. The optimum concentration of promotive action and the minimum concentration of fungistatic action were determined. Results have proved that the steroidal saponin of yam and allum macrostemon bunge has promotive action (in low concentration) and fungistatic action (in high concentration). (C) 2003 Elsevier B.V. All rights reserved.