Objective: The biologic explanation for fetal receptivity to donor engraftment and subsequent long-term tolerance following transplantation early in gestation is not known. We investigated the role fetal immune ontogeny might play in fetal transplantation tolerance in sheep. Methods: Engraftmentof allogeneic and xenogeneicHSC was determined 60 days following transplantation at different time points in sheep fetal gestation. Parallel analysis of surface differentiation antigen expression on cells from lymphoid organs of timed gestational age fetal sheep was determined by flow cytometry using available reagents. Results: An engraftment window was identified after day 52 gestation lasting until day 71 (term gestation: 145 days). This period was associated with the expression of the leukocyte common antigen CD45 on all cells in the thymus. Double-positive and single-positive CD4 and CD8 cells began appearing in the thymus just prior (day 45 gestation) to the beginning of the engraftment window, while single-positive CD4 or CD8 cells do not begin appearing in peripheral organs until late in the engraftment period, suggesting deletional mechanisms may be operative. In concert, surface IgM-positive cells express CD45 in the thymus at day 45, with a comparable delay in the appearance of IgM/CD45 cells in the periphery until late in the engraftment window. Conclusions: These findings support a central role for the thymus in multilineage immune cell maturation during the period of fetal transplantation receptivity. Further, they suggest that fetal engraftment receptivity is due to gestational age-dependent deletional tolerance.