The TK6 and WI-L2-NS human lymphoblast cell lines are derived from the same donor, but WI-L2-NS cells are more resistant to killing by ionizing radiation and also contain a mutation in the coding region of the p53 gene which delays the appearance of apoptosis. To determine whether the difference in radiosensitivity is a result of the slower rate of appearance of apoptosis (referred to as the rate of apoptosis) or to other factors related to the function of p53 such as the G1-phase checkpoint, we have examined the clonogenicity and rates of apoptosis in synchronous populations of these two cell lines as a function of time after exposure to x-rays. The greater radiosensitivity of G1-phase TK6 cells compared to G1-phase WI-L2-NS cells appeared adequate to explain the difference in radiosensitivity of asynchronous cultures. However, TK6 cells irradiated in G1 phase underwent apoptosis about three times more slowly than cells irradiated in other phases of the cycle, cautioning against equating more rapid apoptosis with greater radiosensitivity. The slower rate of apoptosis of G1-phase TK6 cells could not be explained by a radiation-induced block in G1 phase since there was only a short delay in exit of cells from G1 phase. Giant cells (primarily polyploid) were formed after irradiation of WI-L2-NS but not TK6 cells, and fewer giant cells were observed 3-4 days after irradiation in WI-L2-NS cells irradiated in S phase than cells irradiated in other phases. Giant cells were lost from the population through apoptosis which occurred in a synchronous fashion in the multiple nuclei. These results highlight interesting differences in the pattern of cell death between the two cell lines and suggest that the response of TK6 cells in G1 phase, which has the slowest rate of appearance of apoptosis in spite of the fact that it is the most radiosensitive phase for clonogenic survival, may be largely responsible for the reduced shoulder on the radiation survival curve for TK6 cells compared with WI-L2-NS cells.