Excretory/secretory proteins (ESP) from Toxoplasma gondii were analyzed to define the function in the penetration process into host cells. Whole ESP obtained at 37 degrees C were composed of 15 bands with molecular mass of 110, 97, 86, 80, 70, 60, 54, 42, 40, 36, 30, 28, 26, 22, and 19 kDa. Five ESP of 86, 80, 42, 36, and 28 kDa were reacted with monoclonal antibodies (mAb), named as Tg386 (microneme), Tg485 (surface membrane), Tg786 (rhoptry), Tg378, and Tg556 (both dense granules), respectively. The ESP was released by a temperature-dependent/-independent manner and all at once whenever ready to pour out except Tg786. Each ESP was not exhausted within the parasite but the amount was limited. Tg786 was released continuously with increment, whereas Tg378 and Tg556 were ceased to release after 3 and 4 hr. Dense granular Tg378 and Tg556 were released spontaneously and constitutively before the entry into host cells also. The entry of T. gondii was inhibited by all the mAbs differentially. And the parasite deprived of ESP was inhibited to enter exponentially up to 90.1%. It is suggested that ESP play an essential function to provide appropriate environment for the entry of the parasite into host cells.