We report here the investigation on the effects of Achyranthes bidentata polysaccharides (ABPS) against Lewis lung cancer (LLC) in C57BL/6 mice. Depending on its doses administered in vivo, ABPS was shown to have inhibitory as well as stimulative effects on tumor growth in LLC-bearing C57BL/6 mice. ABPS at low dose could significantly inhibit LLC growth, while high dose treatment of ABPS stimulated, rather than inhibited, LLC growth in C57BL/6 mice. Tumor cell cycle analysis revealed that more tumor cells arrested at G2/M phase after daily low dose intraperitoneal injection of ABPS for consecutive 15 days. The spleen weight increased markedly in LLC-bearing C57BL/6 mice treated with high dose of ABPS. However, the spleen cytotoxicity activity was significantly despaired in mice of high dose treatment of ABPS. Furthermore, we demonstrated that the expressions of IL-6 mRNA and TNF-alpha mRNA were markedly up-regulated in spleens from mice treated with a high dose of ABPS by RT-PCR reactions, suggesting that the low dose of ABPS inhibits tumor growth via its effect on tumor cell cycle distribution, rather than activation of NK activity as previously suggested. We postulate that the stimulation of tumor growth by high dose of ABPS is associated with dysfunction of NK cell and up-regulation of IL-6 mRNA and TNF-alpha mRNA expression in murine spleen.