Achyranthes bidentata polysaccharides (ABP), isolated from the root of Achyranthes bidentata Blume, 50 mg.kg-1 ip or 250 mg.kg-1 to ICR mice inhibited the growth of sarcoma 180. ABP 50 and 100 mg.kg-1 ip prolongated the survival days of mice bearing Ehrlich carcinoma. ABP 50-800 micrograms.ml-1 did not exert direct cytotoxic effect in vitro on S180 cells, but enhanced the cytotoxicity of peritoneal macrophages against S180 cells. ABP 50 mg.kg-1 ip x 17 d or 250 mg.kg-1 ig x 16 d promoted the plaque forming cells (PFC) response to sheep red blood cells (SRBC) and serum IgG level as well as splenocyte proliferation induced by mitogen Con A or LPS in tumor-induced immunodeficient mice. ABP also elevated the NK cell activity and serum TNF content in mice bearing S180. These results indicated that the antitumor effect of ABP may be related to its potentiating effect on both specific and nonspecific host immunological responses.