The acquired immunity is underdeveloped at 3-4 weeks of age when piglets are usually weaned on commercial farms, and weaning is associated with compromised immunity. Dietary supplementation with immunomodulatory phytochemicals may enhance immune responses in the weaned piglets. This study is conducted to investigate the effects of dietary supplemental achyranthes bidentata polysaccharide (ABP) on proliferation activity of lymphocytes, and production of antibodies, complements and cytokines in weaned piglets. Results showed that lymphocyte proliferation activity in piglets fed diets supplementing with 1000 and 1500 mg/kg ABP increased (P < 0.05) on days 14 and 28 compared with the non-additive piglets, as well as serum contents of IgG, IgA, IgM, C(3), C(4), IL (interleukin)-2 and IFN (interferon)-gamma. The ABP had dose-dependent immunomodulatory activity and the dose of 1500 mg/kg presented the strongest stimulating activity in vivo. In addition, the ABP increased (P < 0.05) the proliferation activity and production of IL-2 and IFN-gamma of cultured lymphocytes in dose- or time-dependent manner. The proliferation activity of peripheral T cells and splenic lymphocytes in 400 microg/ml of ABP group arrived at their peak values, as well as the production of IL-2 and IFN-gamma at 72 and 12 h after the treatment, respectively. Collectively, these findings suggested that dietary supplementation with ABP to weaned piglets enhances cellular and humoral immune responses, and ABP addition to culture medium also increases the proliferation activity and cytokine production of lymphocytes cultured in vitro, which indicate that dietary supplementation with the herbal polysaccharide may offer an effective alternative to antibiotics for weaned piglets.