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BACKGROUND: Magnetic labeling of stem cells is a recently developed stem cell in vitro labeling technique. Through in conjunction with magnetic resonance imaging (MRI), it can monitor transplanted stem cells in vivo. OBJECTIVE: To identify the method of superparamagnetic iron oxide (SPIO) labeling pig bone marrow mesenchymal stem cells (BMSCs), to investigate the characteristics of stem cells labeled by various SPIO following MRI, and to determine the minimum amount of labeled cells fer MRI. DESIGN, TIME AND SETTING: A control observation was performed at the laboratories of Department of Cardiovascular Surgery, Medical College of Soochow University, and Medical Imaging Centre, First Affiliated Hospital, Soochow University between September 2006 and March 2007. MATERIALS: Fresh porcine iliac bone marrow was collected from Taihu Meishan pigs. SPIO nanometer particles were purchased from Schedng, Germany. Ultramicro SPIO (USPIO) nanoparticles were provided by School of Chemistry and Chemical Engineering, Soochow University. For such particles, crystal nucleus surface was coated with dextran, and following coating, they were named 1#, 2#, 3# for short according to particle size. METHODS: Following isolation, purification, and culture, BMSCs were in vitro labeled by various kinds of SPIO nanoparticles. The labeled cells were subjected to Prussian blue staining and fluorescence microscope observation. The cell growth was observed using MTT method and the growth curve was plotted. For Feridex-labeled cells, 1×106), 5×105, and 1×105 three cell amount groups were set, for unlabeled cells, a 5×105 cell amount group was included, and for 1#, 2#, and 3# SPIO-labeled cells, only 5×105 cell amount group was used. MRI was conduced for measurement of signal intensity of cells labeled by different scanning sequences, followed by statistical analysis. MAIN OUTCOME MEASURES: Detection of SPIO nandparticles-labeled cells by Prussian blue staining; Growth curves of SPIO nanoparticles-labeled cells; Detection of cellular apoptosis by double staining; Determination of signal intensity of cell masses from different Ependoff tubes using MRI with T1WI, T2WI, and fast field echo (FFE) sequences. RESULTS: BMSCs could be labeled with SPIO and the labeling efficiency was 100%. Different amounts of blue-stained Fe particles could be observed in the cytoplasm by Prussian blue staining. SPIO labeling caused a significantly lower signal attenuation effect in T2WI and FFE (T2*WI) images than in T1WI images. In a labeling concentration of 25 mg/L Fe solution, the minimum cell amount for MRI was 1 x 105. The signal intensity exhibited significant difference in 2#, 3#USPIO- and Feridex-labeled cells in no matter T2WI or T2*Wl images(P < 0.01). But no significant difference was found between 1#USPIO- and Feridex-labeled cellss in no mater T2WI or T2*WI images(P > 0.05). There was significant difference in signal intensity of Feridex-labeled BMSCs between T2WI, T2*WI and T1Wl images (P < 0.01). CONCLUSION: BMSCs can be easily and efficiently labeled by SPIO nanoparticles without interference, at proper concentrations on cell viability and proliferation. MRI visualization of SPIO-labeled BMSCs is feasible in both T2WI and T2*WI images.