AIM To investigate the morphological and ultrastructural changes in the human gastric carcinoma cell lineBGC-823 after being treated with tachyplesin.METHODS Tachyplesin was isolated from acid extracts of Chinese horseshoe crab (Tachypleus tridentatus)hemocytes. BGC-823 cells and the cells treated with 2.0 μg/mL tachyplesin were examined respectively withlight microscope, scanning and transmission electron microscope.RESULTS BGC-823 cells had undergone restorative morphological and ultrastructural changes after beingtreated with 2.0 μg/mL tachyplesin. The cells tended to be flat and spread, and their volume enlarged,nucleo-cytoplasmic ratio decreased, the shape of nucleus became relatively regular, the number and volumeof nuleous decreased, heterchromatin decreased while euchromatin increased, the number of mitochondriaincreased with their structure relatively consistent, Golgi apparatus turned to be typical, rough endoplasmicreticulum increased, polyribosome decreased, microvilli and filopodia reduced while lamellipodia increased.CONCLUSION Tachyplesin could change the malignant morphological and ultrastructural characteristics ofhuman gastric carcinoma cells effectively and had certain effects on inducing differentiation of human gastriccarcinoma cells.