A radiation- and chemotherapy-resistant murine osteosarcoma was used to investigate the effect of local hyperthermia (42.5 ± 0.1 °, 30 min) alone and in combination with cyclophosphamide. The cytotoxicity of cyclophosphamide on murine osteosarcoma was established previously in our laboratory. Local hyperthermia (42.5 ± 0.1°, 30 min) had little or no effect on the 16-day-old (206 x 106 osteosarcoma tumor cells/mouse) tumor as shown by the changes in the tumor cell marker, alkaline phosphatase. A 2.5 ± 3.5% reduction in the number of tumor cells was seen. Large tumors treated at 21 days postimplantation (357 x 106 tumor cells) showed a reduction of 24 ± 14%. The effect of combination treatment with cyclophosphamide and hyperthermia produced greater reduction in the numbers of tumor cells than did either treatment used alone. © 1984, American Association for Cancer Research. All rights reserved.