Abstract: Achyranthes bidentata polysaccharides(ABPS) was extracted from the root of Achyranthes bidentata Blume with molecular weight of 1400. It composed of fructose and glucose residues. The molar ratio was 8.7∶1.0. It was a watersoluble neutral polysaccharide of white powder in purity of 99.9%. The effect of ABPS 0.625×10²～2.00×10³ mg·L^-1 on lymphocyte proliferation to Con A on splenocytes from aged mice was evaluated to be about 1.11～3.26 folds in vitro. ABPS 1.00×10³ mg·L^-1 was shown to increase IL-2 production in Con A-stimulated aged mouse splenocytes from 6.7±1.0 u·ml^-1 to 19.3±5.7 u·ml^-1 in vitro. Sandwish-ELISA was used for the determination of TNF-α, TNF-β and sIL-2R in cell suspension. The spectrophotometry with Griess reagent was used for the determination of NO in cell suspension. A method was developed for the determination of NOS activity in biological sample by HPLC. ABPS 50, 100 mg·kg^-1 ip was found to markedly promote TNF-β and NO release, but decrease sIL-2R production from Con A-stimulated aged rat splenocytes, the activity of NOS in those cells was increased. ABPS 100 mg·kg^-1 ip elevated serum TNF-α and NO contents and the NOS activity induced by LPS 0.3 mg·kg^-1 ip in aged rats. ABPS 50～800 mg·L^-1 elevated TNF-α and NO production from PMΦ and increased the NOS activity in PMΦ in vitro, but showed no significant influence on TNF-α, NO release and on NOS activity induced by LPS 10 mg·L^-1 for PMΦ from aged rats. ABPS 100 mg·kg^-1 ip elicited TNF-α, NO production and NOS activity for PMΦ from aged rats, ABPS 100 mg·kg^-1 ip showed a synergetic action with LPS 10 mg·L^-1. When rats were treated with ABPS 50～800 mg·L^-1 ip, there was no significant difference on NO productionon, and NOS activity in the cerebral cortex between those isolated from aged rats and those isolated from young rats. These results indicate that ABPS may prime and trigger MΦ and has restorative effects on the deficiency of the immune system associated with aging in mice and rats.