The immunologic competence of spleen cells of mice, as assessed by their graft versus host capabilities, increases to 35 days of age and beyond. Thymectomy at any point along this continuum of development produces "immunologic arrest;" the peripheral lymphoid tissues of such mice do not show significant increases in activity as the animals mature, nor is there appreciable loss of activity up to 6 months after surgery. Adult spleen cells from mice thymectomized at 1 to 24 hours of age have a greatly reduced ability to induce runt disease. Five million spleen cells from immunologically mature animals will uniformly cause fatal runt disease in neonatal recipients, but this same number of cells from neonatally thymectomized animals produces almost no runt disease. When the dosage of cells from neonatally thymectomized C(57)Bl mice is increased to 20 million, about half of the A recipients develop runt disease. Thus, the defect is a quantitative one. Spleen cells from neonatally thymectomized mice will induce tolerance of skin grafts from the donor strain. In one recalcitrant strain combination, C(57)Bl to A, use of spleen cells from neonatally thymectomized donors as the tolerance-inducing inoculum permits survival of the recipients, which usually die with severe runt disease, but does not induce tolerance. Cell free extracts of spleen and thymus tissue, including "promine" of Szent-Gyorgyi et al., did not affect the runting syndrome or the immunologic reactivity of neonatally thymectomized mice. When syngeneic thymic tissue is grafted into neonatally thymectomized mice, or the animals are given viable syngeneic spleen or thymus cells, the majority of the animals escape the early mortality characteristic of this group. Administration of syngeneic spleen cells from adult donors and grafting of syngeneic neonatal thymus provide restoration of homograft immunity and graft versus host reactivity of the peripheral lymphoid tissues in most of the neonatally thymectomized animals. Thymus cells rarely provide significant restoration of these parameters. Allogeneic thymus grafts also restore neonatally thymectomized mice. Such animals are chimeric: the immunologically competent cells of their peripheral lymphoid tissues are chiefly of host origin as determined by the discriminant spleen assay, but in many instances a significant donor component is also demonstrable in this system. These chimeric animals accept skin grafts from both donor and host strains. A degree of reconstitution has also been attained by grafting of allogeneic adult spleen in neonatally thymectomized animals. The discriminant spleen assay indicates that cells of the donor strain predominate in the peripheral lymphoid tissues of such mice.