The invention relates to a process of magnetic nanoparticles encapsulation in human erythrocytes used as MRI contrast agents. According to the invention, the process consists of erythrocytes dialysis in the presence of iron-based magnetic nanoparticles with sizes of 5…55 nm, suspended in saline solution, for 1…4 h, using 50…75% a SAGM hypotonic medium comprising 150 mM of NaCl, 1.25 mM of adenine, 45 mM of dextrose, 30 mM of mannitol, 2.51 mM of monosodium phosphate, at the temperature of 25°C, while continuously stirring, followed by the recovery of the erythrocytes treated by centrifugation at 600 g and the washing with preservation medium hypertonic solution, to result in human erythrocytes charged with magnetic/superparamagnetic nanoparticles having an encapsulated Fe content of 38.5…69.25%, with an appropriate cell viability, similar to the native erythrocytes.