A study has been made of the dependence of the dose-response curve for mitotic delay on the age of the cells at the time of irradiation. The results obtained for randomly dividing cultures with <tex-math>${\rm H}^{3}{\rm TdR}$</tex-math> used to locate the position of the cells in the cell cycle at the time of irradiation indicated that mitotic delay increases from a value of 0.4 to 0.5 min/rad for cells irradiated in the G 1 phase to a value of 2.3 to 2.4 min/rad for cells irradiated in G 2 . Cells irradiated in a portion of S gave an intermediate value. The number of cells that attempt the first post-irradiation mitosis decreases with dose more rapidly for cells irradiated in S or G 2 than for cells irradiated in G 1 . Irradiation of cells that had been partially synchronized by exposure to FUdR yielded results that did not agree with those obtained with randomly dividing cultures.