[Abstract] Objective: To study the toxicity of intravenous injection of human umbilical cord mesenchymal stem cells in C57/BL6J mice with repeated administrations. Methods: 32 SPF C57/BL6J mice were randomly divided into negative control group and HUMSC transplantation group, of equal gender. HUMSCs were isolated and subsequently cultured through 5 passages in vitro. 2?06 HUMSCs were injected each mice of transplantation group via tail vein once 1 week for 4 weeks, and the equal quantity PBS were injected to control group mice. Mice were euthanatized on the day of 1 week (8 mice per group) and 4 weeks(all survived mice) after the last treatment following the schedule. Toxicity was evaluated by clinical observations, pathology (blood cell counts, clinical biochemistry), immunologic consequences, visceral organs weight and anatomic pathology. Results:No significant difference in hematological and biochemical parameters between HUMSC transplantation group and control group (P > 0.05). There was also no significant change in organ weight or organ coefficient between the two groups(P > 0.05). There was no significant change in the subpopulation of T-cells (the percentage of CD3, CD4, CD8 T cells, and the ratio of CD4 and CD8 T cells). HUMSC transplantation group did not show any histopathological change in the viscera organs compared to control group. Conclusion:Results have suggested that intravenous injection of HUMSCs in C57/BL6J mice with repeated administrations is safe and feasible. No side effects and immune response were found in recipients.