A series monoclonal antibodies were used to study the intrathymic distribution of T and B cells and the process of intrathymic differentiation of T cells on frozen sections of 14 patients' thymuses by a more sensitive immunohistochemical method. The observation indicated that most of the cortical lymphocytes reacted with Leu 1, Leu 2 a, Leu 3 a and Leu 4 antibodies, thus indicating coexpression of multiple antigen on cortical lymphocytes. In cortex, the numbers of Leu 1 and Leu 4 positive cells were less than medulla. In contrary, the numbers of Leu 2 a and Leu 3 a positive cells in the cortex were more than the medulla: and the medullary thymocytes showed segregation into Leu 2 a+ Leu 3 a- and Leu 3 a+ Leu 2 a- subsets. The ANAE+ and AIG 3+ cells were almost limited to the medulla. Hence, the staining profile of medulla approximated the staining pattern of peripheral T cell subsets, which were programmed for their respective role as helper and suppressor cells. Our results supported the conclusion that the mature T cells presented in the medulla were derived from the immature cells in the cortex. In addition, the expression of Leu 7 was not only limited to the NK cells scattered in the medulla, but also found at the cortical-septa border. On the other hand, a small number of B lymphocytes scattered in the medulla stained with Leu 14, OKB-2, BA-1 and IgM antibodies. The Hassall's corpuscles were cross reaction with the B cell antibodies mentioned above; while the epithelial cells in thymic parenchyma were only stained with IgM antibody. Based on these findings we conclude that the human thymus predominantly consist of T cells, but it also contains a fraction of B cells and epithelial cells.