Two sources of adult stem cells that have aroused great interest are human bone marrow-derived mesenchymal stem cells (hMSCs) and human umbilical cord blood cells. hMSCs have been reported to maintain their ability to differentiate into neuronal lineage cells in the central nervous system. Therefore, transplantation of hMSCs represents an attractive new form of cellular therapy for clinical application in spinal cord injury (SCI). The aim of this study was to investigate how transplanted hMSCs from the venous circulation moved into a target zone of compression injury in the spinal cord of rats, and if they ameliorated the behavioral impairments associated with SCI. SCI in rats was induced by compressing the spinal cord for 30 s with an aneurysm clip. hMSCs labeled with cholera toxin subunit B conjugated to fluorescein isothiocyanate (CTX B-FITC) were injected intravenously through the tail vein or directly on the SCI site using a 27-g needle. Suspensions of hMSCs collected from adult humans were delivered at concentrations (1x10(6)cells/200 microliter) in 1 or 5 d after experimental SCI. After transplantation of hMSCs, the SCI regions displayed some endogenous background fluorescence, but CTX B-FITC-labeled hMSCs were clearly identifiable. They were observed in injured but not in intact areas; they were usually round or slightly elongated with a prominent nucleus. Only a few hMSCs were found in the spinal cord in each case but there were more cells in the rats injected at day one than at day five. This study confirmed that these were indeed transplanted hMSCs using antisera recognizing human-specific nuclei or human-specific mitochondria. Double immunofluorescence analysis showed the production of some neuronal and glial cell markers in the SCI lesions. Behavioral test scores of SCI rats treated with hMSCs at day one were significantly better than those for rats treated at day five and for the untreated SCI group. Thus, hMSCs appear to be beneficial in reversing the behavioral effects of SCI in this rat model, even when infused one day after injury. They might be a viable source of stem cells for the treatment of human neurological disorders.