Probiotics and Astragalus polysaccharides (APS) can modulate systemic immunity and intestinal microbiota in animals and human beings. It is still unknown if the combined application of probiotics and APS in feed has synergistic effects on immunity and intestinal microbiota. To address this issue, this study was designed to investigate the synergistic effects on immunity and intestinal microbiota in chicks. A total of 240 female Hy-Line chicks were assigned to 4 treatments. Four treatments were fed the same corn-soy meal control diet; however, treatments 2, 3, and 4 were supplemented (per kg of feed) with 220 mg of APS, 4 x 10(10) cfu probiotics, and dual treatment, respectively. Parameters evaluated included serum Newcastle disease antibody titer, peripheral blood acid alpha-naphthyl acetate esterase-positive (ANAE(+)) T-lymphocyte percentage, immune organ relative weights and histological changes, and selected intestinal tract bacteria. Compared with the control, Newcastle disease antibody titer, ANAE(+) T-lymphocyte percentage, immune organ relative weights, histological changes, as well as lactobacilli and Bacillus cereus numbers significantly increased (P <or= 0.05); however, the Escherichia coli numbers in chicks of the treatments decreased (P <or= 0.05). Interestingly, there were significant differences in ANAE(+) T-lymphocyte percentage of dual treatments compared with APS treatment (P <or= 0.05), and also in spleen index, bursa of Fabricius index, immune organ histological changes, and lactobacilli, Bacillus cereus, and E.coli numbers compared with the APS or probiotics treatments (P <or= 0.05). The results showed that probiotics combined with APS administration in feed displayed synergistic modulation effects on immunity and intestinal microbiota, which is very important for the exploration of new prebiotics.