Radiation-induced DNA damage induction and repair was measured in two human squamous carcinoma cell lines with differing radiosensitive. Experiments were carried out with field inversion gel electrophoresis (FIGE), adapted to measure DNA double strand break (DSB) induction and repair in unlabelled cells. The sensitivity of the method was increased by introducing a hybridization membrane into the agarose gel. Damaged DNA accumulated on one spot on the membrane resulting in high local concentrations. This DNA was quantified using radioactively-labelled total human DNA as a probe. Dose response experiments for damage induction correlated well with the results using prelabelled cells. Linear DNA damage induction curves were observed with a sensitivity for the post-labelling method of 1 Gy. No differences in DSB induction were found, however, between the radiosensitive SCC61 and the radioresistant SQ20B cell line. Repair experiments were carried out with trypsinized cells with different doses and repair temperatures. The 10, 25 and 50 Gy doses resulted in 6, 13 and 50% of the DNA migrating out of the plug at 0 h. For both the cell lines 75-85% of the initial damage was repaired within 1 h at 37 degrees C at all three radiation doses, i.e. no significant differences were observed in repair rates or extent between the two cell lines. At 24 degrees C repair was slower than at 37 degrees C, and at 0 degree C no repair was observed. In summary, radiosensitivity differences at physiological temperatures could not be explained by differences in either induction or repair of DNA damage as measured by pulsed field gel electrophoresis.