Ferrimagnetic microspheres 20-30 microm in diameter are useful as thermoseeds for inducing hyperthermia in cancers, especially for tumors located deep inside the body. The microspheres are entrapped in the capillary bed of the tumors when they are implanted through blood vessels and heat cancers locally by their hysteresis loss when placed under an alternating magnetic field. In the present study, preparation of magnetite (Fe(3)O(4)) microspheres 20-30 microm in diameter was attempted by melting powders in high-frequency induction thermal plasma, and by precipitation from aqueous solution. The microspheres prepared by melting powders in high-frequency induction thermal plasma were composed of a large amount of Fe(3)O(4) and a small amount of wustite (FeO), and those subsequently heat treated at 600 degrees C for 1 h under 5.1 x 10(3) Pa were fully composed of Fe(3)O(4) 1 microm in size. The saturation magnetization and coercive force of the heat-treated microspheres were 92 emu g(-1) and 50 Oe, respectively. The heat generation of the heat-treated microspheres was estimated to be 10 Wg(-1), under 300 Oe and 100 kHz. The microspheres prepared by precipitation from aqueous solution consisted of beta-FeOOH, and those subsequently heat treated at 400 degrees C for 1 h in a 70% CO(2) + 30% H(2) atmosphere consisted of Fe(3)O(4) crystals 50 nm in size. The saturation magnetization and coercive force of the heat-treated microspheres were 53 emu g(-1) and 156 Oe, respectively. The heat generation of the heat-treated microspheres was estimated to be 41 Wg(-1), under 300 Oe and 100 kHz. The latter microspheres are believed to be promising thermoseeds for hyperthermic treatment of cancer.