The development of lymphocytes within the fetal and neonatal BALB/c mouse thymus is reviewed with particular emphasis on the maturity of immunologic functions. Fetal thymocytes respond by vigorous proliferation to stimulation by allogeneic lymphoid cells or by phytohemagglutinin. Such reactivity is much diminished in neonatal thymus or thymic-derived (T) cells in neonatal spleen. Splenic T cells seem to mature more slowly than immunoglobulin-bearing B lymphocytes in the neonatal spleen, but the finding is confounded by the presence of large numbers of "suppressor" T cells in the neonatal spleen. For example, the in vitro antibody response to the T-independent antigen dinitrophenyl-lysine-Ficoll is optimal by 2 or 3 weeks of age, but the in vitro response to T-dependent sheep erythrocytes does not reach adult levels until 6 weeks of age, suggesting a deficiency in T "helper cells." The response of neonatal spleen cells to sheep erythrocytes cannot be reconstituted by adult T cells however, unless neonatal splenic T cells are first depleted by anti-Thy 1 serum and complement. The target of this T suppressor cell seems to be only B cells, and not other T cells. The overall sequence of T lymphocyte maturation in the mouse seems to start with large numbers of reactive T cells as well as some functionally active helper or effector T cells in early neonatal life, and finally to achieve a stable equilibrium between T cell subpopulations between 5 and 6 weeks of age.