Cell fusion, premature chromosome condensation (PCC) and conventional cytogenetics were used to test whether the biochemical process of chromatin condensation-decondensation throughout the cell cycle, which depends on cyclin-regulated histone H1 kinase activity, affects the conversion of DNA damage into chromosome damage and determines intrinsic cell cycle-stage radiosensitivity. Results from three sets of experiments are presented. Irradiated G0 human lymphocytes were fused to exponentially growing hamster cells and time allowed for repair, while following the hamster cells in their progress towards mitosis. Severe fragmentation was observed in the induced lymphocyte PCCs when hamster cells entered mitosis 13 h after irradiation, suggesting conversion of DNA damage into non-repairable chromosome damage during G1/S transition. When PCC was used to analyse chromosome damage directly in G0 and G2 phase lymphocytes, the induction of breaks per cell per chromatid per Gy was found to be similar, suggesting that G2 increased radiosensitivity is related to chromatin condensation occurring during G2/M transition and not to an inherent chromatin structure at this phase. When chromatin condensation-decondensation at the G1/S and G2/M transitions was modified after irradiation by using conditioned media or elevated temperature (40 degrees C), a dramatic change in the yield and the type of chromosomal aberrations was observed. All results obtained were consistent with the proposed hypothesis. They may be also helpful in the characterization of a DNA-chromosome damage conversion process which could give a biochemical explanation of the variability in radiosensitivity observed at the various stages of the cell cycle as well as among mutant cells and cells of different origin. The proposed conversion process is cell cycle-regulated and, therefore, subject to up-regulation or down-regulation following mutagen exposure and genetic alterations.