Measurement of the surviving fraction after 2 Gy (SF2) may predict for local control of the tumour and patients cure, but clonogenic assays are unsuitable for wider clinical application. Promising results have been obtained using DNA damage assays such as pulsed-field gel electrophoresis, PFGE. In the current study, nine human tumour cell lines (SF2, range 0·08-0·62) were studied for DNA double-strand break (dsb) induction and six of these for dsb rejoining using PFGE. Differences in dsb induction, as the slope (±SEM) of DNA release per Gy, varied from 1·30 (0·05) to 2·42 (0·17). The dsb induction frequency varied from 3·55 (0·33) to 9·69 (2·18) dsb × 10-9/bp/Gy (21-56 dsb/Gy/cell). Variations in the half-time for fast phase (18-60 min) and slow phase (38-445 min) dsb rejoining were observed. Statistically significant correlations were found between SF2 and the slope of the DNA release curve (p = 0·003), DNA release after 10 Gy (p = 0·029) and 20 Gy (p = 0·011) and slow phase dsb rejoining (p = 0·012). While the underlying mechanisms of cell killing remain unclear, PFGE measurement of dsb induction and rejoining shows great potential as a predictive assay for intrinsic cellular radiosensitivity.