HeLa-S3 cells were treated with heat and/or radiation in medium of pH 7.4 or pH 6.7. A heat treatment at 44 degrees C for 10 min, which alone did not produce any measurable cell killing, was found to enhance the lethal effect of radiation to the same extent at pH 6.7 and 7.4. Heat alone at 44 degrees C for 30 min killed 65 per cent of cells at pH 7.4 and 90 per cent at pH 6.7 and produced a marked radiation sensitization of the survivors but only slightly more at pH 6.7 than at pH 7.4. These results support the suggestion that heat damage responsible for increase in sensitivity to subsequent irradiation is distinguishable from damage which leads to cell death after heat alone, and that heat used as a radiation sensitizer may only give a small therapeutic gain in cancer treatment, based on the lower pH of some tumour cells.