Sixty chickens were randomly divided into two groups to determine the effect of oral administration of probiotics on the intestinal mucosal immune response and ultrastructure of cecal tonsils. The first group (control) was fed with a basic diet without antibiotic or probiotics. The second group was fed with the same diet as the control, except they received drinking water with probiotics (4 x 10(9) cfu per chicken and day) from posthatch to day 3 of age. The probiotic preparation was composed of Bacillus subtilis Bs964, Candida utilis BKM-Y74 and Lactobacillus acidophilus LH1F. Intestinal fluid, Peyer's Patch and cecal tonsils were taken at day 1, 4, 7, 10 and 18 after administration of probiotics. The results showed: (i) Compared to the control, probiotics enhanced the content of following items: immunglobulin (Ig)A in the intestinal fluid at day 7 (p < 0.01), the IgG-forming cells at day 10 (p < 0.05), IgM-forming cells in the Peyer's Patch at day 7 (p < 0.05), IgA-forming cells at day 7-10 (p < 0.05), IgG-forming cells at day 7 (p < 0.05) and IgM-forming cells in cecal tonsils diffuse area at day 4-7 (p < 0.05). (ii) T lymphocytes in cecal tonsils were enhanced at day 7 (p < 0.01) after orally fed with probiotics. (iii) The density of microvilli and length of cecal tonsils increased after probiotics were administrated at day 3. With chicken ageing, the efficiency of probiotics would decrease. These results suggested that probiotics enhance intestinal mucosal immunity of chicken at the early age.