In the course of ontogeny, the homing site for the hematopoietic stem cells (HSC) moves with certain predictability from the yolk sac to the liver/spleen and then to the marrow. The pattern of this migration has thus far been established mostly on a morphologic basis. To delineate further the course of this migration and to gain insight into its possible mechanism, we used in utero transplantation of allogeneic or xenogeneic HSC in preimmune sheep fetuses. Sex chromosome, type of hemoglobin, and species-specific surface markers were used to follow the path of transplanted cells in the fetus. Before the development of the bone marrow, transplanted HSC (liver- or marrow-derived) homed exclusively to the liver/spleen. With the development of marrow, around day 60 of gestation (term, 145 days), homing occurred also in the nascent marrow and by day 80 transplanted cells homed exclusively to the marrow. This suggests that there may be a hierarchy in homing sites, with those of the marrow having higher affinity than those of liver/spleen. Interestingly, despite a change in homing that was followed by the expansion of the marrow compartment of HSC (ie, HSC proliferation), these cells did not participate actively in blood cell formation during most of the prenatal period. Liver remained the major hematopoietic organ throughout the gestation. It was only during the perinatal period that this organ assumed the function of hematopoiesis from the liver. This lack of expression of HSC in fetal marrow can possibly be attributable to the immaturity of marrow stroma required for differentiation and maturation of progenitors and the orderly egress of mature cells into the blood stream. The availability of this model allows us to begin studies in the molecular mechanism of stem cell homing in vivo during ontogeny.