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The rapid development of biomimetic cell membrane-based nanoparticles is still overshadowed by many practical challenges, one of which is the difficulty to precisely measure the biodistribution of such nanoparticles. Currently, this challenge is mostly addressed using fluorescent techniques with limited sensitivity, or radioactive labeling methods, which rarely account for the nanoparticles themselves, but their payloads instead. Here we report the development of a robust method for the innate radioactive labeling of cells and membrane-based nanoparticles and their consequent sensitive detection and biodistribution measurements. The preclinical potential of this method was demonstrated with Nano-Ghosts (NGs), manufactured from the cytoplasmic membranes of mesenchymal stem cells cultured with radioactively-labeled linoleic acid and achieving a cell labeling efficiency of 36%. Radiolabeling did not affect the physiochemical properties of the NGs, which stably retained their radiolabels. Using radioactivity measurements, we are now able to determine precisely the amount of NGs uptaken by tissues and cells, thereby providing further support to our presumed active NG targeting mechanisms. Biodistribution studies comparing radiolabeled NGs to fluorescently-labeled ones have validated our method and revealed new information, which could not be obtained otherwise, regarding the NGs' unique kinetics and rapid clearance, supporting their excellent safety profiles. The reported approach may be expanded to other membrane-based entities to facilitate and hasten their preclinical development and be used in parallel with other labeling methods to provide different and additional information.