We developed extremely small functionalized magnetic nanoparticles (MNPs) for use as an in vivo delivery system for pharmaceuticals and biomolecules. We functionalized the MNPs (d = 3 nm) by silanization of amino groups on the particles with (3-aminopropyl)triethoxysilane for subsequent cross-linking with pharmaceuticals and biomolecules. The MNPs were successfully introduced into living cells without any further modification, such as the use of cationic residues, to enhance endocytic internalization. The particles could be incorporated into the subcutaneous tissue of a mouse's ear through the skin of the ear and could be localized by application of an external magnetic field.We also developed a cell-specific delivery system that makes use of MNPs (d = 3 nm) conjugated with folic acid and a coumarin fluorophore for recognition by folate receptors on the cell surface. The modified MNPs were internalized by human pharyngeal cancer cells (KB cells) after an incubation time that was short compared with the time required for internalization of MNPs without folic acid. Cellular recognition of MNPs may lead to the development of other cell-specific delivery systems.These functionalized MNPs are expected to be useful as a new drug delivery tool.