The murine fetal thymus obtained early in gestation was found to develop normally in a syngeneic host. In contrast to the adult thymus grafts, the fetal thymus did not undergo the early and marked necrosis following transplantation into syngeneic hosts. Indeed, the fetal thymus was as effective as the adult thymus in achieving immunologic reconstitution of neonatally thymectomized syngeneic mice, and superior to adult thymus, when transplanted into allogeneic hosts. Furthermore, the fetal thymus, unlike the adult thymus, did not induce graft-vs-host disease (GVH) in genetically susceptible neonatally thymectomized mice. These observations seem relevant to thymus transplantation in children with a congenital absence of the thymus. Transplantation of fetal thymus in a child with DiGeorge syndrome corrected the T-cell immunodeficiency, and led to a reduction toward normal of the B-cell population.