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As a freshwater pearl mussel, Hyriopsis cumingii is widely cultured in China to produce pearls. This study was made to explore the antitumor activity of water-soluble polysaccharide (WSP) from mantles of H. cumingii. Cell viability of human hepatocarcinoma HepG2 cells was estimated by MTT method. Cell cycle analysis was determined by flow cytometry. Apoptosis was observed under fluorescence microscopy and confirmed by DNA fragment assay. Reverse transcriptase-polymerized chain reaction (RT-PCR) and immunocytochemistry were carried to evaluate c-myc, bcl-2 and cyclinD1 gene expressions in HepG2 cells. Furthermore, intracellular reactive oxygen species (ROS) production was assessed by flow cytometry. After WSP treatment, the growth of HepG2 cells was inhibited and most of cells arrested in G(0)/G(1) phage (p < .01); apoptotic changes in nucleus and cytoplasm were also observed in WSP-treated cells; percentage of apoptotic rate significantly increased in a dose-dependent manner (p < 0.001); DNA fragmentation was detected in treated HepG2 cells; The mRNA level and protein level of c-myc, bcl-2 and cyclinD1 were decreased in the treated HepG2 cells. ROS was significantly increased in a dose- and time-dependent manner as well. The results suggested that WSP has potent antitumor activity.