Ferromagnetic polyaniline (PANI) with conductivity was synthesized with peroxydisulfate as an oxidant and horseradish peroxidase as a catalyst in an N'-a-hydroxythylpiperazine-N'-ethanesulfanic acid buffer solution containing aniline, HCl, and NiCl2-6H(2)O in an applied magnetic field. The result of an electron paramagnetic resonance spectrum indicated that there were unpaired electrons in the resulting product, the spin density of which was 7.60 x 10(19) spins/g. The curve of the magnetization versus the magnetic field showed that PANI had soft ferromagnetic behavior at about 300 K. The saturation magnetization. and coercive force of PANI were 0.033 emu/g and 5 Oe, respectively. Ultraviolet-visible and Fourier transform. infrared spectra indicated that there was interaction between Ni2+ and PANI chains but the structure of the backbone chains of PANI synthesized in the presence of a magnetic field hardly changed compared with that of PANI synthesized without NiCl2-6H(2)O. (c) 2008 Wiley Periodicals, Inc.