Human thymus tissue was examined from 7 wk of gestation through birth for the expression of antigens reacting with a panel of anti-T cell monoclonal antibodies. Additionally, the reactivities of reagents against the transferrin receptor, against leukocytes, against low m. w. keratins, and against major histocompatibility complex antigens were studied on human fetal thymic tissue. Frozen tissue sections were evaluated by using indirect immunofluorescence assays. At 7 wk of gestation, no lymphoid cells were identified within the epithelial thymic rudiment; however, lymphoid cells reacting with both antibody 3A1, a pan T cell marker, and antibody T200, a pan leukocyte reagent, were identified in perithymic mesenchyme. After lymphoid colonization of the thymic rudiment at 10 wk of fetal gestation, fetal thymic tissue reacted with antibodies T1, T4, and T8. At 12 wk of gestation, antibodies T3, T6, A1G3 (anti-p80, a marker of mature thymocytes), and 35.1 (anti-E rosette receptor) all reacted with thymic tissue. Our findings indicate that T cell antigens were acquired sequentially on thymocytes at discrete stages during the first trimester of human fetal development. The 3A1 antigen was present on fetal lymphocytes before lymphoid cell colonization of thymic epithelium, suggesting that passage through the thymus was not required for the expression of the 3A1 antigen by T cell precursors. The appearance of mature T cell antigens, T3 and p80, on thymocytes by 12 wk of gestation implies that the T cell antigen repertoire may be established in the thymus during the first trimester. Thus, a critical period of T cell maturation appears to occur between 7 and 12 wk of human fetal gestation.