An experiment was conducted to evaluate the effects of dietary supplementation of a polysaccharide isolated from Astragalus membranaceus (APS) on performance and immune responses in weaned pigs. A total of 144 crossbred pigs weaned at 26 to 30 days of age with an average initial live weight (LW) of 7·64 (s.d. 0·290) kg were randomly allotted to six diets supplemented with APS at 0, 100, 250, 500, 750, and 1000 mg/kg. There were six replicates (three barrow pens and three gilt pens) per diet treatment with four pigs per pen. Pigs were given food ad libitum for 21 days and the LW and food intake were measured on days 14 and 21. Pigs were intramuscularly injected with 1 mg/kg LW ovalbumin (OVA) on day 14 to evaluate humoral immune response. Blood samples were collected on day 21 to measure leukocyte differential counts, percentage of blood CD4+ and CD8+ lymphocyte subsets, lymphocyte proliferation response to Concanavalin A, serum concentration of immunoglobulin G (Ig G), interleukin-2 (IL-2), interleukin-4 (IL-4), interleukin-10 (IL-10), interferon-γ (IFN-γ) and specific OVA antibody. The results showed that the average daily gain, the numbers of WBC and lymphocytes, the proportion of CD4+ lymphocyte subset, and the contents of IL-2 and IFN-γ increased ( P < 0·05) as pigs were fed increased supplemental level of APS during the 21 d period. However, the contents of specific OVA antibody, Ig G, IL-4, and IL-10 were not affected ( P > 0·05) by dietary levels of APS. The broken line analysis and quadratic regression analysis indicate that the optimal APS supplemental level would be between 381 mg/kg and 568 mg/kg for the maximal ADG and from 324 to 563 mg/kg for immune responses. Collectively, this study suggests that dietary APS can be used as a potential immuno-modulating agent by affecting cellular immunity of weaned pigs.