The study investigates the effects of x-ray irradiation on the reproductive capacity of single HeLa cells, derived from a human cervical carcinoma. The researchers developed a method to grow single mammalian cells into macroscopic colonies with 100% plating efficiency, enabling precise studies similar to those conducted on bacteria. Experiments showed that x-irradiation reduces the reproductive capacity of HeLa cells, leading to slower growth and the formation of abortive colonies. The survival curve for HeLa cells was found to have a shoulder around 75 r, after which the survival rate decreases logarithmically, with a D₀ value of 96 r, indicating a 37% reduction in survivors for each additional 96 r of dose. Cells that are killed by irradiation may still divide a limited number of times, forming microcolonies, or may become giant cells that do not reproduce. Giant cells are highly susceptible to viral destruction and can be isolated in large numbers. The study also suggests that the lethal effect of x-rays is due to a genetic defect rather than a simple single gene mutation. The survival curve indicates a multiple hit mechanism, with two hits required for cell death. The results show that HeLa cells are highly sensitive to radiation, with a D₀ value much higher than that of many microorganisms. The study highlights the importance of understanding radiation effects on mammalian cells, as the sensitivity of HeLa cells suggests that radiation may play a significant role in determining the lethal dose for whole-body irradiation in mammals. The findings also have implications for understanding the development of radiation syndromes and the potential for viral infection in irradiated cells.The study investigates the effects of x-ray irradiation on the reproductive capacity of single HeLa cells, derived from a human cervical carcinoma. The researchers developed a method to grow single mammalian cells into macroscopic colonies with 100% plating efficiency, enabling precise studies similar to those conducted on bacteria. Experiments showed that x-irradiation reduces the reproductive capacity of HeLa cells, leading to slower growth and the formation of abortive colonies. The survival curve for HeLa cells was found to have a shoulder around 75 r, after which the survival rate decreases logarithmically, with a D₀ value of 96 r, indicating a 37% reduction in survivors for each additional 96 r of dose. Cells that are killed by irradiation may still divide a limited number of times, forming microcolonies, or may become giant cells that do not reproduce. Giant cells are highly susceptible to viral destruction and can be isolated in large numbers. The study also suggests that the lethal effect of x-rays is due to a genetic defect rather than a simple single gene mutation. The survival curve indicates a multiple hit mechanism, with two hits required for cell death. The results show that HeLa cells are highly sensitive to radiation, with a D₀ value much higher than that of many microorganisms. The study highlights the importance of understanding radiation effects on mammalian cells, as the sensitivity of HeLa cells suggests that radiation may play a significant role in determining the lethal dose for whole-body irradiation in mammals. The findings also have implications for understanding the development of radiation syndromes and the potential for viral infection in irradiated cells.