We report that ultra-small, monodisperse, water-dispersible magnetite (Fe(3)O(4)) nanoparticles can be synthesized by a facile one-pot approach using trisodium citrate as crystal grain growth inhibitor and stabilizer in polyol solution. The resultant Fe(3)O(4) nanoparticles exhibit an excellent long-term colloidal stability in various buffer solutions without any modification. They are also superparamagnetic at room temperature and their magnetic property relies heavily on their size. Due to the low magnetization and good water-dispersibility, the 1.9 nm-sized Fe(3)O(4) nanoparticles reveal a low r(2)/r(1) ratio of 2.03 (r(1) = 1.415 mM(-1) s(-1), r(2) = 2.87 mM(-1) s(-1)), demonstrating that they can be efficient T(1) contrast agents. On the other hand, because of the excellent magnetic responsivity, the 13.8 nm-sized Fe(3)O(4) nanoparticles can be readily modified with nitrilotriacetic acid and used to separate the protein simply with the assistance of a magnet. In addition, these Fe(3)O(4) nanoparticles may be useful in other fields, such as hyperthermia treatment of cancer and targeted drug delivery based on their size-dependent magnetic property and excellent stability.