Abstract: By immunohistochemistry,the CD3 ＋ T lymphocytes and CD4 ＋ and CD8 ＋ T cell subsets in chicken cecal tonsil,including appearance,migration,tissue localization and changes of the number,were determined by applying monoclonal antibodies of CD3,CD4 and CD8.The results showed that CD3 ＋,CD8 ＋ T lymphocytes first appeared on 11 day-old embryo,while CD4 ＋ T lymphocytes in the 15th day.Changes in distribution,CD3 ＋ cell smainly in the mucosal epithelium,CD4 ＋ cells in the mucosa in the lamina propria,but CD8 ＋ cells in the lamina propria and mucosal epithelium.Changes in the amounts,the number of positive cells were all increased dramatically before the 7th day,and at the 7th day,CD3 ＋ cells significantly decreased in the number,but CD4 ＋,CD8 ＋ T cells had no significant changes in the number.From the 21st day to the 35th day,CD3 ＋,CD4 ＋ and CD8 ＋ T lymphocytes in the number increase continuously.The conclusion could be made that the immune function can be enhanced in the early post-hatched chicken duodenum.