Survival and G2 delay of L929 mouse fibroblasts exposed to 3.4-MeV alpha particles depend on the cell age at the time of irradiation. Greatest sensitivity for both endpoints has been found at the G1/S transition: The surviving fraction of G1/S cells is reduced to 0.11 following 1 Gy of alpha particles compared to 0.31 for early G1 cells. The G2 + M transit time rises from 3 hr for control cells to 22 and 30 hr for cells irradiated with 0.3 Gy in G2 or at the G1/S boundary, respectively. Cells irradiated in early G1 do not show increased G2 + M transit times. Growth delay as calculated for the entire population increases linearly with dose by 23 hr/Gy of alpha particles.
