For cellular MR imaging, conventional approaches to intracellular magnetic labeling of nonphagocytic cells rely on the use of secondary compounds such as transfection agents and prolonged incubation of cells. Magnetoelectroporation (MEP) was investigated as an alternative method to achieve instant (<1 s) endosomal labeling with the FDA-approved formulation Feridex, without the need for adjunct agents or initiating cell cultures. While MEP was harmful at higher voltages or pulse durations, the procedure could be properly calibrated using a pulse of 130 V and 17 ms. Labeling was demonstrated for stem cells from mice, rats, and humans; the uptake of iron was in the picogram range and comparable to values obtained using transfection agents. MEP-labeled stem cells exhibited an unaltered viability, proliferation, and mitochondrial metabolic rate. Labeled mesenchymal stem cells (MSCs) and neural stem cells (NSCs) differentiated into adipogenic, osteogenic, and neural lineages in an identical fashion as unlabeled cells, while containing Feridex particles as demonstrated by double immunofluorescent staining. MEP-labeled NSCs proliferated normally following intrastriatal transplantation and could be readily detected by MR imaging in vivo. As MEP circumvents the use of secondary agents, obviating the need for clinical approval, MEP labeling may be ideally suitable for bedside implementation. Magn Reson Med, 2005. © 2005 Wiley-Liss, Inc.