SU MMARYThe present study was conducted to determine the effects of a polysaccharide of Atractylodes macrophala Koidz (PAM) as a dietary additive on growth performance, immunoglobulin concentration and IL-1b expression in weaned piglets. One hundred and twenty LandracerYorkshire piglets weaned at 28 days old (body weight 7 . 5¡0 . 07 kg) were assigned to five treatment groups (three pens/ group, eight piglets/pen) fed maize/soybean-based diets supplemented with 0, 3, 6 or 9 g of PAM/kg diet or antibiotics (0 . 4 g flavomycin/kg+0 . 13 g olaquindox/kg). The experimental period was 28 days. With increasing PAM supplementation levels, average daily gain was greater (quadratic, P<0 . 05) and the ratio of amount fed to live weight (LW) gain (feed/gain) improved (quadratic, P<0 . 05) during days 14-28 and overall, and diarrhoea incidence decreased (linear, P<0 . 05) during days 14-28. Supplementation of PAM also increased (quadratic, P<0 . 05) serum concentrations of interleukin (IL)-2 and IL-6 on day 14, and increased (quadratic, P<0 . 05) IL-1b expression in jejunal mucosa and lymph nodes. Concentrations of PAM between 6 and 9 g/kg presented the strongest bioactivity compared to the control group or antibiotic-fed group. These findings indicate that PAM is effective in improving growth performance and cytokine response, which suggests that PAM can be used as a diet additive for weanling piglets.