Magnetic nanoparticles that display high saturation magnetization and high magnetic susceptibility are of great interest for medical applications. Magnetite nanoparticles display strong ferrimagnetic behavior and are less sensitive to oxidation than magnetic transition metal nanoparticles such as cobalt, iron, and nickel. For in vivo applications, well-defined organic coatings are needed to surround the magnetite nanoparticles and prevent any aggregation. The goal of this research was to develop complexes of magnetite nanoparticles coated with well-defined hydrophilic polymers so that they could be dispersed in aqueous fluids. Focal points have included the following: (1) Investigations of polymer systems that bind irreversibly to magnetite at the physiological pH, (2) the design of block copolymers with anchor and tail blocks to enable dispersion in biological fluids, and (3) investigations of copolymer block lengths to maximize the concentration of bound magnetite. Hydrophilic triblock copolymers with controlled concentrations of pendent carboxylic acid binding groups were designed as steric stabilizers for magnetite nanoparticles. These copolymers were comprised of controlled molecular weight poly(ethylene oxide) tail blocks and a central, polyurethane anchor block containing carboxylic acids. Stoichiometric aqueous solutions of FeCl2 and FeCl3 were condensed by reaction with NH4OH to form magnetite nanoparticles, and then a dichloromethane solution of the block copolymer was added to adsorb the copolymer onto the magnetite surfaces. Stable magnetite dispersions were prepared with all of the triblock copolymers. The polymer-nanomagnetite conjugates described in this paper had a maximum saturation magnetization of 34 emu/g. Magnetization curves showed minimal hysteresis. Powder X-ray diffraction (XRD), selected area electron diffraction (SAED), and high-resolution electron microscopy (HREM) confirmed the magnetite crystal structure. Transmission electron microscopy (TEM) showed that the dispersions contained magnetite particles coated with the polymers with a mean diameter of 8.8 +/- S.D. 2.7 nm.