Human inherited diseases could possibly be cured for life if the patient received, and did not reject, healthy precursor cells specific for his disease. This cell transfer or transplantation treatment can occur in utero before a baby is even born. In this study, we inserted human blood-precursor cells into the abdominal cavity of mice with the aim of finding human blood cells now being made inside the mouse such that the mouse’s body no longer rejects a foreign human skin graft. We found that the blood precursor cells remained in the mouse’s abdominal cavity for months after transplant, fortifying the idea that this cavity can be a useful site for effective transplantation.