The response of an experimental fibrosarcoma to combined heat and X rays has been assessed using delay in tumour regrowth. No thermal sensitization was observed for one hour of heating at 41.5 degrees C. A thermal enhancement ratio of 1.4 to 1.5 was seen for one hour of heating at 42.5 or 43.5 degrees C immediately after irradiation. The importance of the sequence and time interval between the two modalities was tested using a constant heat treatment of 42.5 degrees C for one hour. Heating was most effective when given after irradiation, with little change in TER for an interval of a half to six hours. When heat preceded the irradiation the pattern was more complex, with no sensitization at half, one, and six hours, but with a significant effect at two and three hours. The degree of sensitization often depended upon the X-ray dose, being lower below X-ray doses of about 20 Gy. The therapeutic advantage of the combined treatment was assessed by comparison with previously published results for skin reactions. No therapeutic benefit was found for treatments when heat and X-rays were separated by one hour or less. A therapeutic gain factor of about 1.3 was observed for heat given two to six hours after irradiation of this fibrosarcoma.