DNA neutral (pH 9.6) filter elution was used to measure radiation-induced DNA double-strand break (dsb) frequencies in eight human squamous cell carcinoma cell lines with radiosensitivities (D0) ranging from 1.07 to 2.66 Gy and D values ranging from 1.46 to 4.08 Gy. The elution profiles of unirradiated samples from more radiosensitive cell lines were all steeper in slope than the profiles from resistant cells. The shapes of the dsb induction curves were curvilinear and there was some variability from cell line to cell line in the dose-response for the induction of DNA dsb after exposures to 5-100 Gy 60Co gamma-rays. There was no relation between the shapes of the survival curves and the shapes of the dose-responses for the induction of DNA dsb. At low doses (5-25 Gy), three out of four of the more sensitive cell lines (D less than 2.5 Gy) had larger initial break frequencies than the more resistant lines (D greater than 3.0 Gy). Although the low-dose (5-25 Gy) elution results were variable, they do suggest that DNA neutral elution will detect differences between sensitive and resistant tumour cells in initial DNA dsb frequencies.