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Mice with transplanted Sarcoma 180 were irradiated 2 hours daily; 1st through 14th day after transplantation with 3-GHz microwaves at a power density of 40 mW/cm2. The irradiation resulted in an increase of rectal temperature by 3 to 4 °C. Some animals that were exposed to radiation received daily an IP injection of 1 mg/kg of Colcemide (synthetic Colchicine) or of 0.5 mG/kg of purified Streptolysin S or of both compounds. On the 14th day after transplantation half of each group of mice received 1 μCi/g IP 3H-thymidine; the other halves 3H-uridine, 1 μCi/g IP. Three hours later all of the mice were killed; the tumors were dissected, then measured for mass and radioactivity. Mean tumor mass, number of regressed tumors and averaged counts of radioactivity were calculated for each group. Microwave hyperthermia resulted in reduction of tumor mass by about 40%; the reduction was enhanced when microwave hyperthermia was combined with Colcemide and/or Streptolysin S. In the combined treatment with all three factors the tumor mass was reduced by about 70%. Inhibition of the tumor's growth was accompanied by reduced uptake of 3H-thymidine and 3H-uridine into the tumorous tissues.