Fetal liver haematopoietic cells were used to induce neonatal tolerance to cardiac allografts. Newborn mice were injected with fetal or newborn haematopoietic liver cells in two fully allogeneic strain combinations. There was no clinical evidence of graft-versus-host disease. Long-term survival of subsequent cardiac allografts occurred in both strains without immunosuppressants. Unresponsiveness was found not be be donor-specific with prolongation of third-party allografts as well as donor-type grafts. These findings have important implications for inducing tolerance in paediatric organ transplantation.