Encapsulating magnetic nanoparticles within red blood cells is one strategy for extending the lifetime of magnetic resonance imaging contrast agents in the bloodstream. Human red blood cells were incubated for 12 hours with iron oxide (gamma-Fe2O3) nanoparticles with a broad range of particle and aggregate sizes (ranging from 10 to 600 nm) at different osmolarities ranging from 100 to 290 mOsm before being returned to an osmolarity of 300 mOsm. Concentrations of nanoparticles trapped within the cells were measured using transmission electron microscopy and iron-mapping by electron energy loss spectroscopy. An osmolarity of 200 mOsm was found to be the optimal condition for loading of the cells with nanoparticles. At this osmolarity, it was shown that the concentration of particles within the cells relative to the average concentration in the suspension is maximized. At 200 mOsm, the maximum size aggregate of particles that entered the cells was approximately 120 nm.