Objective To investigate the growth inhibition and the apoptosis inducement effects of polysaccharide of trametes robiniophila murr (PS-T) on human rectal cancer cell line HR8348 and elucidate the possible mechanisms. Methods After treatment with PS-T, the growth inhibition rate of human rectal cancer cell strain HR8348 was studied by MTT method. The apoptotic index was detected by methyl green and pyronine Y staining and by terminal deoxynucleotidyl transferase(TdT) mediated dUTP nick end labeling (TUNEL). The bcl-2, bcl-xl, bax, bak and p53 gene expressions of HR8348 cells were examined by immunohistochemical method.Results PS-T induced a dosedependent inhibition of HR8348 cells in vitro. The maximum percentage of growth inhibition was 71.1% by 36 h after administration of PS-T at a concentration of 4.0 mg/ml. There was no significant difference in the inhibitory rate as compared to the positive control (5-FU 10 μg/ml, P gt;0.05). The typical apoptosis phenomenons were observed under the light microscope by both staining methods. The apoptotic index increased parallelling with the increase of concentration of PS-T. When the PS-T concentration was 4.0 mg/ml, the apoptotic index determined by methyl green and pyronine Y staining and by TUNEL method increased rapidly to 0.1620±0.0128 and 0.2612±0.0158, respectively, which was greater than that of the positive control (5-FU 10 μg/ml, Plt;0.05). The expression of bcl-2, bcl-xl, bak and p53 was increased in the PS-Ttreated HR8348 cells by 36 h, while the bax remained unchanged. Expression index of bak/bcl-2 and bak/bcl-xl was increased significantly as compared with the control (Plt;0.05). Conclusion PS-T can significantly inhibit growth and induce apoptosis of human rectal cancer cell strain HR8348 in vitro. The apoptosis induced by PS-T might be related to the increase of the ratio of bak to bcl-2 and to bcl-xl and upregulation of the expression of p53 gene.