Achyranthes bidentata polysaccharides (ABP), extracted from the root of Achyranthes bidentata Blume, 25-100mg.kg-1.d-1 x 7 could inhibit tumor growth by 31%-40%. Combination of cyclophosphamide (Cy) and ABP increased the rate of tumor growth inhibition to 58%. ABP 50 and 100% mg/kg ip could potentiate LAK cell activity and increase the Con A (5 micrograms/ml)-induced production of tumor necrosis factor (TNF-beta) from murine splenocytes. The optimal time for TNF production was on d 8. We also found that ABP 1-2 micrograms/ml strongly inhibited the proliferation of S180 and K562 cells in vitro. The S180 cell membrane content of sialic acid was increased and phospholipid decreased after ABP acting on cells for 24 hours. The changes were significantly different from the control group (P < 0.05 or P < 0.01), but the membrane cholesterol content and membrane mobility indices (Ch/PI) were not affected. The results suggest that the antitumor mechanism of ABP may be related to potentiation of the host immunosurveillance mechanism and the changes in cell membrane features.