Liver cells or thymic grafts from homozygous embryonic mice were subjected to passage through appropriate nonoperated or thymectomized adult F1hybrids. Passage through a nonthymectomized host was required for maturation of potential immunologically competent cells derived from embryonic liver; this was not true with regard to cells derived from a fetal thymic graft. Cells from embryonic liver were found first in the host thymus while those from thymic grafts appeared to migrate exclusively to peripheral lymphoid tissues.