Abstract: Objective To study the mechanism of the hepatotoxicity of microcystion-UR.Methods MTT test and flow cytometry were applied in the research on the effects of microcystin-LR on the cell growth,cell cycle and apoptosis of primary cultured hepatocyte and Hela cell strain.Results Microcystic-LR can inhibit cell growth at very low dose and can promote cell proliferation at a relatively higher dose.Typical changes of apoptosis as well as and cell cycle block and synchronization at the G₂/M-phase induced by mcirocystine-LR were observed.The results suggested that microcystin could affect hepatocyte cell cycles and induce cell proliferation as well as apoptosis.Similar but less distinct effect on Hela cell strain were detected.Conclusion It seemed that the special effects of microcystins on hepatocyte could be one of the mechanisms of hepatotoxicity of microcystins.