Superparamagnetic iron oxide nanoparticles (SPIONs) are commonly used in magnetic resonance imaging (MRI), but their fast phagocytosis makes them less than ideal for this application. To circumvent the lymphocyte-macrophage system, we encapsulated SPIONs into red blood cells (RBCs). For loading, the RBC's membrane was opened by swelling under hypoosmotic conditions and subsequently resealed. In this work, we demonstrate that SPIONs can be loaded into RBCs in a concentration sufficient to obtain strong contrast enhancement in MRI.