Pedigrees of HeLa S 3 cells irradiated with 300 rads of X-rays were studied by time-lapse photography. The pedigrees were analyzed by classifying the cells as surviving and nonsurviving groups. In both groups, high degree of synchrony in division was observed after a division delay. In the nonsurviving group, loss of cells occurred in a synchronous fashion. Generation time was restored to nearly normal in the 1st postirradiation generation, but prolonged in succeeding ones. Division probability in the surviving group approached unity (0.9-1.0) except for a slightly reduced value (0.74) at the 3rd generation. In the nonsurviving group, the division probability was relatively high in the irradiated and the 1st generations (0.76), but declined from the 2nd to 4th generations with a progressive increase in cell damage, such as fusion and death.