Purpose: To assess the feasibility of loading iron oxide nanoparticles in endothelial microparticles (EMPs), thereby enabling their noninvasive monitoring with magnetic resonance (MR) imaging in mice.Materials and Methods: Experiments were approved by the French Ministry of Agriculture. Endothelial cells, first labeled with anionic superparamagnetic nanoparticles, were stimulated to generate EMPs, carrying the nanoparticles in their inner compartment. C57BL/6 mice received an intravenous injection ofnanoparticle-loaded EMPs, free nanoparticles, or the supernatantof nanoparticle-loaded EMPs. A 1-week follow-up was performed with a 4.7-T MR imaging device by using a gradient- echo sequence for imaging spleen, liver, and kidney and a radial very-short–echo time sequence for lung imaging. Comparisons were performed by using the Student t test.Results: The signal intensity loss induced by nanoparticle-loadedEMPs or free nanoparticles was readily detected within 5 minutes after injection in the liver and spleen, with a more pronounced effect in the spleen for the magnetic EMPs. The kinetics of signal intensity attenuation differed for nanoparticle-loaded EMPs and free nanoparticles. No signal intensity changes were observed in mice injected with the supernatant of nanoparticle-loaded EMPs, confirming that cells had not released free nanoparticles, butonly in association with EMPs. The results were confirmed by using Perls staining and immunofluorescence analysis.Conclusion: The strategy to generate EMPs with magnetic properties allowed noninvasive MR imaging assessment and follow-up of EMPs and opens perspectives for imaging the implications of these cellular vectors in diseases.