Fetal allotransplant recipients are resistant to graft-versus-host disease

•Injection of allogeneic T cell-replete bone marrow induces graft-versus-host disease (GVHD) in the neonatal mouse, whereas the fetal mouse is resistant.•Compared to the neonatal serum, the fetal serum contains elevated levels of interleukin-10, which is conducive to regulatory T cell induction.•Resistance to GVHD can be transferred from the fetal in utero hematopoietic cell transplantation recipient to the neonate by adoptive transfer of regulatory T cells.