Abstract: Objective To study the development of T and B lymphocytes in human fetal palatine tonsil. Methods Totally 22 samples of human fetal palatine tonsil from the 9th to 20th gestational week (GW) were collected. T and B lymphocytes were displayed with cluster of differentiation（CD)，anti-CD3 antibody and anti-CD20 antibody by immunohistochemical ultrasensitive-SP method. The immunoreactive positivecells were counted with BioMiaspro photographs analysis software and analysised with statistics software.Results In the 9th GW, there were rare CD3-positive cells and CD20-positive cells, distributed in the mesenchyme under the mucous membrane epithelium. In the 14th GW, the amounts of CD20-positive cells were significantly increasing, part of them were converged in primary lymphoid nodule,others were located in the diffused lymphoid tissue and crypt epithelium. At the same time, the amounts of CD3-positive cells were still to remain less, but a few of them were also infiltrated into the crypt epithelium.In the 17th GW, the number of primary lymphoid nodule was increaseing. In the 20th GW, the quantities of CD3-positive cells were significantly increasing, which were diffused distribution and formed grouping in the lymphoid tissue of lamina propria. The two types of positive-cells were increasing gradually with fetal age growing（EM>P/EM><0.05 or EM>P/EM><0.01）,but the numbers of CD20-positive cells were majority (EM>P/EM><0.05). Conclusion In the 9th GW, T and B cells have emerged in the human fetal palatine tonsil; In the 14th GW, B cells converged and

Key words: T cell, B cell, CD3, CD20, Palatine tonsil, Immunohistochemistry, Human fetus