We have followed with autoradiography the course of DNA synthesis and with cytofluorometry the partition of DNA during mitosis, through the different phases of the life-span of human fibroblasts. Towards the end of the proliferative potential of the cell population, DNA synthesis was disturbed during the late part of the S period. The DNA content of metaphases and of anaphases had a normal Gaussian distribution during the whole proliferative life-span of the cell population. At the very end, however, when cells entered the post-mitotic phase, the distribution of DNA content deviated from the normal Gaussian probability, becoming significantly skewed.
