Oxidative polymerization of m-phenylenediamine was carried out using H2O2 as an oxidant and horseradish peroxidase as a catalyst in mixtures of aqueous buffer solution and 1,4-dioxane or in reversed micellar solutions. When the reaction mixture was brought into contact with a stainless steel stick, the obtained polymer responded to a permanent magnet at room temperature in the air. From the ICP emission spectroscopic analysis, it was found that these polymers contained small amounts of transition metals, such as Fe and Ni. The magnetic properties of the obtained polymers were studied with a superconducting quantum interference device (SQUID). The M (magnetization)-H (magnetic field) curves showed sigmoid behavior at 300 K, but did not exhibit a residual magnetization and a coercive force. These properties resembled those of soft ferromagnetic materials. The ferromagnetic behavior was indicated even at 800 K. Polymers of o- and p-phenylenediamines exhibited lower values of saturation magnetization than polymers of m-phenylenediamine.