New rhenium radiolabeled compounds are of general interest due to their nuclear characteristics which allow radiotherapy and in situ monitoring of tumor uptake. Biocompatible magnetic nanoparticles capable of transporting radionuclides, providing MRI contrast agent properties for imaging and a therapeutic effect in the target tissue simultaneously, are highly desirable. Herein we describe the preparation of magnetoferritin samples, and their labeling with rhenium in the form of the low oxidation state rhenium(I)-tricarbonyl complex, [Re(CO)(3)(H2O)(3)](+). A non-radioactive rhenium isotope (Re-187) was used in all studies. The rhenium complex was conjugated covalently to the surface lysine groups of the protein cage via glutaraldehyde crosslinker and histidine modification. The analyses of conjugates were performed by inductively coupled plasma mass spectroscopy (ICP-MS) and size exclusion chromatography (SEC). Labeling efficiency was calculated as 22 +/- 2 rhenium per protein cage. The in vitro stability of the rhenium carbonyl label was evaluated at room temperature and in human serum medium. It was found that 91.1 +/- 1.8% rhenium was retained on the surface of the magnetoferritin cage following 72 h of dialysis. Prussian blue staining revealed the uptake of rhenium labeled nanocages preferentially into the human breast metastatic adenocarcinoma, MDA-MB-231 cells lines. The cytotoxicity assay carried out with the same cell lines showed that there is no significant cytotoxic effect up to 72 hours of incubation with 1 mg of labeled nanocages per mL (IC50 value).