We have developed a ferromagnetic bone cement as a thermoseed to generate heat by hysteresis loss under an alternate magnetic field. This material resembles bioactive bone cement in composition, with a portion of the bioactive glass ceramic component replaced by magnetite (Fe3O4) powder. The temperature of this thermoseed rises in proportion to the weight ratio of magnetite powder, the volume of the thermoseed, and the intensity of the magnetic field. The heat-generating ability of this thermoseed implanted into rabbit and human cadaver tibiae was investigated by applying a magnetic field with a maximum of 300 Oe and 100 kHz. In this system, it is very easy to increase the temperature of the thermoseed in bone beyond 50 degrees C by adjusting the above-mentioned control factors. When the temperature of the thermoseed in rabbit tibiae was maintained at 50 to 60 degrees C, the temperature at the interface between the bone and muscle (cortical surface) surrounding the material rose to 43 to 45 degrees C; but at a 10-mm distance from the thermoseed in the medullary canal, the temperature did not exceed 40 degrees C. These results demonstrate that ferromagnetic bone cement may be applicable for the hyperthermic treatment of bone tumors.