PURPOSE: In vivo magnetic resonance (MR) tracking of magnetically labeled bone marrow mesenchymal stem cells (BMSCs) administered via the mesenteric vein to rats with liver fibrosis. MATERIALS AND METHODS: Rat BMSCs were labeled with superparamagnetic iron oxide (SPIO) and the characteristics of the BMSCs after labeling were investigated. Eighteen rats with CCL4-induced liver fibrosis were randomized to three groups to receive SPIO-labeled BMSCs (BMSC-labeled group), cell-free SPIO (SPIO group), or unlabeled BMSCs (control group). MR imaging of the liver was performed at different time points, and signal-to-noise ratio (SNR) of the liver was measured. In vivo distribution of delivered BMSCs was assessed by histological analysis. RESULTS: Labeling of BMSCs with SPIO did not significantly alter cell viability and proliferation activity. In BMSC-labeled group, the liver SNR immediately decreased from 8.56+/-0.26 to 3.53+/-0.41 at 1 h post injection and remained at a significantly lower level till 12 days (P<.05 versus the level before). By contrast, the liver SNR of the SPIO group almost recovered to the preinjection level (P=.125) at 3 days after a transient decrease. In control group, the liver SNR demonstrated no significant difference at the tested time points. Additionally, Prussian blue-positive cells were mainly distributed in the liver parenchyma, especially in injured areas. CONCLUSION: The magnetically labeled BMSCs infused through the mesenteric vein can be detected in the fibrotic liver of rats using in vivo MR imaging up to 12 days after injection.