Cell progression into mitosis and chromatid aberration frequencies were compared in two Chinese hamster ovary (CHO) cell lines after incorporation of 125IdUrd. Asynchronous, exponentially growing populations of CHO K1 and the DNA repair-deficient, radiation-sensitive CHO irs-20 cells were compared after a 10-min exposure to 14.8 kBq/ml 125IdUrd. Essentially no differences were seen for either end point between the cells of the two cell lines. As the cells in S phase at the time of labeling entered the mitotic cell selection window, the number of mitotic cells of each cell line declined to approximately 60% of the respective unlabeled control. Chromosome analysis of the mitotically selected cells indicated an 125I decay-dependent increase in the number of chromatid aberrations in cells of both cell lines. The appearance of aberrations together with the known rates of production and rejoining of DNA double-strand breaks show that cells are able to progress through G2 phase and into mitosis in the presence of such breaks. The data suggest that DNA damage may be necessary, but is not sufficient to cause a radiation-induced blockade of cell progression through G2 phase.