The neutral filter elution method and the neutral comet assay have been used to analyze radiation-induced DNA damage and repair in 6 human tumor cell lines: HT-144 melanoma; DU-145 prostate carcinoma; U-87 glioma; WiDr and HT-29 colon adenocarcinomas; and SiHa cervical carcinoma. In spite of large differences in intrinsic radiosensitivity measured using a clonogenic assay, double-strand break induction, rejoining rate, and amount of residual DNA damage 4 h after irradiation were similar among these cell lines when measured using the neutral comet assay. Differences in initial numbers of double-strand breaks were observed using the neutral filter elution method; however, there was no correlation with radiosensitivity, nor did the rejoining rate or amount of residual DNA damage measured using filter elution correlate with radiation sensitivity. We conclude that neither double-strand break assay is able to reliably rank cells according to clonogenic survival following irradiation.