Astragalus membranaceus (AM) has been widely used for treating kidney diseases in traditional Chinese medicine. In this study, the Astragalus polysaccharide (APS), the main active ingredient was isolated and purified from the Rhizomes of AM, which consisted of d-glucopyranose and had the molecular weight of 3.6x10(4) Da. The effect of APS on glomerulonephritis rats induced by cationic Bovine Serum Albumin(C-BSA) was evaluated by flow cytometry using Nuclear Transcription Factor-kappaB (NF-kappaB) as marker. Interleukin-2 (IL-2), Interleukin-6 (IL-6) and Tumor Necrosis Factor-alpha (TNF-alpha) were determined by the ELISA method. The rats (model group and treatment group) were injected subcutaneously with C-BSA plus incomplete Freund's adjuvant on day 0, C-BSA was injected through the caudal vein from week 2 to week 7 to induce glomerulonephritis. The rats (treatment group) were given APS by intraperitoneal injection from week 2 to week 7. The expression of NF-kappaB and the concentration of IL-2, IL-6 and TNF-alpha were significantly decreased in the treatment group. This study clearly suggests that APS is effective in protecting against glomerulonephritis induced by C-BSA through the inhibition of NF-kappaB mediated-cytokine pathway.