Phenotype and functions of cells of the T lymphocyte lineage from fetal liver, thymus, spleen and bone marrow were investigated at various ages. T lymphocyte differentiation was shown to be initiated in the thymus after the 7th week of gestation. In this organ, a large number of cells with a phenotype comparable to that of children thymocytes and with a high proliferative response to phytomitogens was observed at the 14th week. The fetal liver and bone marrow never contained many T-cells and the liver was shown to be virtually devoid of any of these cells before the 13th week. Fetal spleen contained appreciable amounts of T-cells after the 13th week. Helper and suppressor activities of fetal thymocytes and splenocytes were acquired between the 12th and the 16th week, but they were never as complete nor as potent as those of adult lymphocytes. HLA antigens were detected in very low amount in lymphocytes from the various organs at the beginning of the second trimester and their expression was significantly enhanced by in vitro incubation with alpha-interferon (alpha-IFN), a procedure that permits easier HLA typing of fetal cells.