For the purpose of separating microorganisms from culture broth by magnetic force, magnetic particles were conjugated with a polymer to produce polymer-conjugated magnetite (polymer-mag). Among 4 preparation methods investigated-aminosilane coupling, glycidylsilane coupling, crosslinking, and co-precipitation-, polymer-mag prepared by co-precipitation showed the highest cell recovery and high dispersibility. When various cationic, anionic, and nonionic polymers were used to prepare polymer-mag and applied to the magnetic separation for Escherichia coli, magnetite conjugated with chitosan (chitosan-mag) gave the highest cell recovery. In addition, E. coli cells could be recovered as the precipitant only 1 min after chitosan-mag was added to a cell suspension, and a clarified supernatant was obtained. The amount of E. coli cells adsorbed to the chitosan-mag was about 1 g-dry cells/g-chitosan-mag, and cell recovery of over 90% was attained in wide pH range from 3.0 to 7.0. Of 12 microorganisms tested, 4 could be recovered with chitosan-mag at recovery levels above 90%, and the adsorbed amounts exceeded 0.5 g-dry cells/g-chitosan-mag. Differences in adsorbed amounts were considered to be mainly due to the different zeta potential of the microorganisms tested.