OBJECTIVE To evaluate the early hematopoietic reconstitution of implanted SCID mice which were transplanted with human umbilical cord blood-derived mononuclear cells (UCB-MNC) and placenta-derived mesenchymal stem cells (PMSC) by intro-bone marrow injection (IBMI). METHODS The placenta tissues were digested by collagenase IV and cultured with low-glucose DMEM supplemented with b-FGF. The adherent cells were collected and passaged. The phenotypes of the cultured cells were detected by flow cytometry. The osteogenic differentiation and adipogenic differentiation were induced and detected. SCID recipient mice conditioned with sublethal dose irradiation were transplanted with human UCB-MNC and PMSC by IBMI or intravenous injection (IV). Fifty recipient mice were divided into five groups at random: cotransplantation group A (PMSC+UCB-MNC by IBMI), single transplantation group B (UCB-MNC by IBMI), cotransplantation group C (PMSC by IBMI, UCB-MNC by IV), normal saline control group D (normal saline by IBMI), normal control group E (normal saline by IBMI). There were ten recipient mice in every group. On day 14, the bone marrow cells of every recipient mouse were flushed out from the injected tibias and contralateral tibias, respectively. The percentage of human CD34+ and CD45+ hematopoietic cells was analyzed by flow cytometry. RESULTS PMSC were isolated and expanded from human placenta, which were successfully induced to osteogenic differentiation and adipogenic differentiation. FACS analyses showed that the phenotypes of PMSC were normal. On day 14 after transplantation in SCID mice, the percentages of human CD34+ and CD45+ hematopoietic cells in the tibias of group B were both significantly lower than them in the injected tibias and contralateral tibias of group A. CONCLUSIONS Human PMSC could enhance the early engraftment of UCB-MNC in SCID mice.