Previous reports have suggested that measuring radiosensitivity of normal and tumor cells would have significant clinical relevance for the practice of radiation oncology. We hypothesized that radiosensitivity might be predicted by analyzing DNA end-binding complexes (DNA-EBCs), which form at DNA double-strand breaks, the most important cytotoxic lesion caused by radiation. To test this hypothesis, the DNA-EBC pattern of 21 primary human fibroblast cultures and 15 tumor cell lines were studied. DNA-EBC patterns were determined using a modified electrophoretic mobility shift assay and were correlated with radiosensitivity, as measured by SF2. DNA-EBC analysis identified a rapidly migrating ATM-containing band (identified as "band-A") of which the density correlated with SF2 (0.02 </= SF2 </= 0.41) in primary fibroblasts (r(2) = 0.77). The DNA-EBC pattern of peripheral blood lymphocytes was identical to that of fibroblasts. In addition, band-A density correlated with SF2 (0.35 </= SF2 </= 0.80) in 15 human tumor cell lines (r(2) = 0.91). Densitometry of other bands, or total DNA-EBC binding, correlated more poorly with SF2 (r(2) < 0.45). These data indicate that DNA-EBC analysis may be a practical, clinically relevant predictor of tumor and primary cell radiosensitivity.