[en] The manifestation of DNA damage at the nucleoid level was examined in several AT cell lines using an image analysis system to directly visualize and measure the changes in DNA loop size which occur when increasing concentrations of propidium iodide (PI) are used to titrate the DNA supercoiling response (the 'fluorescent halo assay'). This response consists of a relaxation (0.5 - 7.5 μg/ml PI) and rewinding phase (10-50 μg/ml PI), the latter of which is impaired by the presence of DNA strand breaks in irradiated cells. Results may reflect a greater instability of DNA-nuclear matrix attachment points in irradiated AT fibroblasts. The DNA supercoiling response in irradiated transformed AT fibroblasts and AT lymphoblasts did not differ from that observed in unaffected cells of the same type, but all immortalized cell lines (AT and unaffected) had inherently larger DNA loop sizes than diploid fibroblasts and exhibited excess unwinding after irradiation. (author)