A new triterpenoid saponin, named justicisaponin-I, has been isolated from Justicia simplex D. Don. (Acanthaceae). It has been assigned oleanolic acid-3-O-beta-D-glucopyranosyl-4'-O-ferulate structure (1) on the basis of chemical transformation and comprehensive spectral evidence. Justicisaponin-I has produced significant sperm acrosomal membrane stabilizing action and may prove to be a potent antifertility agent.