The four kinds of plant polysaccharides, i.e., pachyman polysaccharides (PPS), Acanthopanax senticosus polysaccharides (ASPS), polysaccharides of tremella fuciformis (TF) and lentinan, have obviously inhibitory action against the animal tumor growth and have been applied to the treatment of cancer. The mechanism was that they could enhance the body immune function, but whether the tumor cells were killed is not clear. In this paper, the effects of the four plant polysaccharides on cell proliferation in mice sarcoma (ascitic type) S180 and human chronic myelogenous leukemia K562 cells were studied with MTT chromometry. Tt was found That TF and lentinan had no effect on both cell line, but PPS and ASPS could obviously inhibit the proliferation of them, the IC50 of PPS was 1.5mg/ml in both cell line, that of ASPS was 0.38 mg/ml (S180 cells) and 0.28mg/ml (K562 cells) respectively. This result indicated that the PPS and ASPS were able to kill the tumor cell directly. To investigate the mechanism of antitumor action of PPS and ASPS, the sialic acid (SA), phospholipid (PI) and cholesterol (Ch) contents of S180 cell membrane were examined after the PPS or ASPS application for 24 hours. No significant changes were observed for the Ch and Ch/Pl ratio, the amount of SA increased and that of PI lowered respectively (P < 0.05). The results suggested that the antitumor action of PPS and ASPS not only related to the action of enhancing the body immune function but also related to the changes of cell membrane.