Current knowledge of human T cell ontogeny is reviewed in terms of appearance of T cells in central and peripheral lymphoid organs, maturation of T cell markers, and development of immune functions. Extrapolation of growth curves derived from cell counts from fetal thymus, spleen, and bone marrow indicates the appearance of lymphocytes at 3.5 weeks gestation. E-rosette-forming cells are present in thymus at 11 weeks and in peripheral organs 15 to 16 weeks gestation. beta-2-Microglobulin is associated with all lymphoid cells by 13 weeks gestation. Lymphocyte responses to the T cell mitogen phytohemagglutinin (PHA) are first detected in thymus at 10 weeks and in spleen and blood 3 to 4 weeks later. Allogeneic responses in mixed lymphocyte reactions develop at about 7.5 weeks in fetal liver and later in thymus and peripheral organs. Lymphocytotoxicity for xenogeneic cells is a property of bone marrow cells and not thymocytes. Several aspects of development of a suppressor T cell in human neonates is discussed and related to similar findings in the mouse. These studies indicate a relatively high degree of maturation of human T cells during fetal life.