Time-lapse cinemicrographs of synchronous HeLa S3 cells irradiated with 220 kv X-rays at various stages of interphase provided data for constructing pedigrees, measuring the duration of both generation cycles and mitoses, and scoring events associated with cell disintegration for up to seven postirradiation generations. The onset of the first mitosis after doses of 500 rads was delayed as expected from previous studies of the age dependence of "mitotic delay." The interval between this first mitosis and the next was indistinguishable from that for unirradiated control cells, while the subsequent two generations were again prolonged, on the average, though not so severely as was the irradiated generation. The duration of mitosis was increased proportionally more than interphase. Cell disintegration took place by way of two morphologically distinct processes. In three-quarters of the cases the cells were rounded and apparently trapped in metaphase when they disintegrated; the remaining disintegrations occurred in spread, interphase cells. In cells disintegrating from the rounded configuration, the generation preceding disintegration was prolonged relative to that in cells which divided; in cells disintegrating from either configuration, the penultimate generation was also prolonged. The mitotic times were disproportionately increased in both of these generations. It is suggested that in this system X-ray damage is preferentially expressed as derangement of the mitotic process; such damage ultimately brings about permanent mitotic arrest in the majority of cells.