Fluorescence in situ hybridization (FISH) is a potential assay for determining cellular radiosensitivity based on the detection of chromosome damage. This approach was chosen because of its relative simplicity and short assay time. Two radiosensitive and two radioresistant human tumour cell lines were used. The radiosensitive lines were an ovarian carcinoma line (A1847) and a squamous carcinoma line (SCC61). The radioresistant cells were a lung adenocarcinoma line (A549) and a second squamous line (SQ20B). Whole chromosome-specific probes were used to detect radiation-induced chromosome aberrations in mitotic cells. Available probes were first screened to characterize the intrinsic chromosome aberrations before irradiation and the appropriate probes (minimum fluorescent spots) were selected for each cell line. Maximum radiation-induced aberrations were found 24 h after irradiation. Dose-response curves corrected for target size (proportion of genome probed) differed for all cell lines. The radiosensitive A1847 cell line showed more induced aberrations compared with the radioresistant A549 cell line, in agreement with the survival data. In contrast, the SQ20B cell line showed more induced chromosome aberrations than the more radiosensitive SCC61 cell line, leading to the hypothesis that the SQ20B cells could tolerate more aberrations. Dose-response curves obtained in surviving cells 14 days postirradiation indeed showed elevated levels of chromosome aberrations for SQ20B cells. The difference in chromosome aberrations between 1 and 14 days showed a good correlation with the survival data for all four cell lines. In conclusion, FISH of mitotic cells with whole chromosome probes appears to be a suitable assay to predict radiosensitivity. It seems necessary, however, to determine both induced and remaining chromosome aberrations, since different processing or tolerance of radiation-induced aberrations, including stable types, could lead to different correlations with cell survival.