The immunomodulatory and immunosuppressive capacity of human mesenchymal stem cells (hMSC) is well recognized, but efficacies of hMSC in immunocompetent and immunocompromised animals have never been directly compared.We aimed to compare the efficacy of hMSC in preventing bleomycin-induced lung injury in immunocompromised SCID and immunocompetent C57Bl/6 mice.SCID and C57Bl/6 mice were subjected to a single bolus intranasal instillation of bleomycin to induce lung injury. One million hMSC were administered intravenously 24 h following the induction of bleomycin lung injury.hMSC xenotransplantation into SCID mice resulted in transient improvements in lung weight and tidal volume and to persistent improvement in inspiratory duty cycle, inspiratory flow rate and inspiration/expiration ratio. We did not observed protective effects in C57Bl/6 mice. This correlated with histological changes, where hMSC administration reduced Ashcroft scores, collagen deposition and inflammatory influx in the lungs of SCID mice, but not in those of C57Bl/6 mice.The application of hMSC for the treatment of acute and chronic lung injury is significantly affected by the immune status of the recipient. Lack of hMSC-mediated repair observed in C57Bl/6 mice was likely to be due to limitations of their immune privilege and differential priming of hMSC in immunocompetent versus immunocompromised hosts.