Polymer-coated magnetite fine particles with carboxyl groups were developed for applications as an immunoassay magnetic marker. The magnetite composites were prepared using the following method. A hydrophilic macromer with a polymerizable vinyl group at the terminal was adsorbed on the surface of the magnetite fine particles. Subsequently, a hydrophilic vinyl monomer with a carboxyl group and a crosslinking agent were added to carry out a copolymerization reaction. The resulting magnetite composite particles were dispersed stably in an aqueous medium. The composite particles exhibited high antibody binding ability and high-sensitive magnetic properties suitable for applications as magnetic markers.