Nano-dispersed cerium dioxide is promising for use in medicine due to its unique physicochemical properties, including low toxicity, the safety of in vivo usage, active participation in different redox processes occurring in living cells, and its regenerative potential, manifested in the ability of CeO2 to participate repeatedly in redox reactions. In this work, we examined the biological activity of cerium dioxide nanoparticles (CeO2 NPs) synthesized by precipitation in mixed water/alcohol solutions at a constant pH of 9. This synthesis method allowed controlling the size and Ce3+/Ce4+ proportion on the surface of NPs, changing the synthesis conditions and obtaining highly stable suspensions of "naked" CeO2 NPs. Changes in the surface properties upon contact of CeO2 NPs with protein-rich media, e.g., bovine serum albumin and DMEM cell culture medium supplemented with 10% fetal bovine serum, the characteristics of nanoparticle uptake by mouse aortic endothelial cells and the antioxidant activity of the nanoparticles of different sizes were investigated by various state-of-the-art analytical methods.