BACKGROUND: T. vaginalis is a pathogenic protozoa infecting genitourinary tract. Metronidazole is a drug of choice to treat T. vaginalis infection. However, because of appearance of metronidazole- resistant T. vaginalis, it is needed to develop an alternative drug. So, we evaluated the effects of various concentration of kalopanaxsaponin A against T. vaginalis. METHODS: The antiprotozoal effect of kalopanaxsaponin A on the growth and fine structure of T. vaginalis was examined by using trypan blue exclusion assay and electron microscopy. RESULTS: The viability test showed markedly decreased number of T. vaginalis, treated with increasing concentration of kalopanaxsaponin A. We evaluated the electron microscopic findings for antiprotozoan effects against T. vaginalis. SEM showed that in T. vaginalis treated with 4 microgram/mL for 1 hour, axostyle was shrinked and flagella began to disappear. With gradual increase of the concentration of kalopanaxsaponin A, destruction of cell membrane was getting more severe, and degenerative change was observed in T. vaginalis treated with 8 microgram/mL for 2 hours. TEM showed that in T. vaginalis treated with 4 microgram/mL for 2 hours, the vacuoles in cytoplasm were larger and hydrogenosomes were smaller than those in control group. The number of vacuole was increased, the nucleus was destroyed, the number of polyribosome and free ribosome was also decreased in trichomonads treated with kalopanaxsaponin A (8 microgram/mL), which indicated the occurrence of degenerative changes. CONCLUSION: These results indicated that kalopanaxsaponin A had the antiprotozoal effect on T. vaginalis by inhibition of cell multiplication as well as an impairment of protein synthesis.