The response of six types of mouse tumour to single doses of X rays alone or to X rays in combination with moderate hyperthermia (42.5 degrees C/60 min) has been assessed using delay in tumour regrowth. Thermal sensitization was observed in five of the six tumours. The degree of sensitization varied with the size of the X-ray dose, being larger at higher doses. The degree of sensitization also depended on the sequence and separation of the heat and irradiation. The thermal sensitization has been measured in terms of the X-ray doses to produce the same level of tumour damage with or without heat, i.e. thermal enhancement ratios. These TER values, measured for X-ray doses in excess of 20 Gy, are not greater in any of the tumours than in a range of normal tissues, if the X rays and heat are given in close succession. Separation of the heat and X rays reduces the TER values slightly, but some effect is still apparent at 3--24 hours. In normal tissues the effect of heat is totally lost within four hours. Comparison of these tumour data with published normal tissue data indicates a therapeutic advantage if the heat and X rays are separated by more than one hour. This therapeutic gain is most reliably achieved and heat given after irradiation.