Various radiation responses in mammalian cells depend on the position of the cell within its generation cycle (that is, its age) at the time of irradiation. Studies have most often been made by irradiating synchronized populations of cells in vitro. Results in different cell lines are not easy to compare, but an attempt has been made here to point out similarities and differences with regard to cell killing and division delay. In general, survival data obtained so far show that, in cells with a short G₁, cells are most sensitive in mitosis and in G₂, less sensitive in G₁, and least sensitive during the latter part of the S period. In cells with a long G₁, in addition to the above, there is usually a resistant phase early in G₁ followed by a sensitive stage near its end. (The latter may be as sensitive as mitosis.) Exceptions to the above, especially in some L cell sublines, have been noted, and a possible explanation is given.

In Chinese hamster cells, maximum survival after irradiation occurs during S, but it does not coincide with the time of the maximum rate of DNA synthesis or with the time of the maximum number of cells in DNA synthesis, and changes in survival also occur in cells inhibited from synthesizing DNA. Rather, survival depends on the position the cell has reached in the cycle at that time, which involves not only DNA synthesis but other processes as well. Survival is not completely correlated with DNA synthesis, since halting DNA synthesis just before or just after irradiation only slightly affects survival at its maximum.

Division delay exhibits a pattern of response which is similar in most cell lines. Delay is considerable for cells irradiated in mitosis, is small for cells in G₁, increases to a maximum for cells during S, and declines for cells in G₂. L cells or human kidney cells may have a longer delay for cells irradiated in G₂ than for those irradiated in S. The results can be explained in terms of a two-component model of division delay. One component results from the prolongation of the S period due to the reduced rate of DNA synthesis, and the other, a block in G₂, is independent of DNA synthesis. The proportion of the two components may vary in different cell lines.