Recent data suggest that the differences in radiosensitivity between cell lines can be related to differences in dsb induction (Radford 1986a). In the light of this we have set out to assess the extent to which differences in radiation survival between human tumour cell lines can be attributed to differences in dsb induction. For nine human tumour lines survival was assayed by clonogenic assay and compared with dsb induction by irradiation at ice-bath temperature as measured by neutral filter elution. The lines varied widely in their sensitivity, ranging from a sensitive neuroblastoma (surviving fraction at 2 Gy, SF2 = 0·13) to a resistant bladder carcinoma (SF2 = 0·62). Dsb induction was found to vary between the cell lines, such that resistant cells generally suffered less damage than sensitive ones. However, the relationship between damage induction and celluar sensitivity was not a simple one, and other factors which may influence sensitivity need to be invoked. These data suggest that, in human tumour cell lines, differences in radiosensitivity may at least in part be due to different levels of damage induction, but that some lines may vary in their tolerance of damage due to differences in biological characteristics such as repair capacity.