Human osteosarcoma cells (HuO-3Nl cells) and fibroblastic cells with normal human karyotype (HuO-3 cells) were derived from a patient with osteosarcoma. HuO-3N1 cells were more sensitive to heat treatment than HuO-3 cells. The combination of heat and cis-dichlorodiammineplatinum (II) (CDDP) exerted synergistic cytotoxity on HuO-3N1 cells, and was much less cytotoxic to HuO-3 cells than to HuO-3Nl cells. Therefore, thermochemotherapy with CDDP may be useful for treatment of human osteosarcoma.