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 ${\rm H}^{3}{\rm TdR}$ 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 G1 phase to a value of 2.3 to 2.4 min/rad for cells irradiated in G2. 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 G2 than for cells irradiated in G1. 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.