The physical and chemical actions of ionizing radiation are characterized by great local energy release but nonspecific general action. From one point of view, ionizing radiation should have very little effect on most cells. It is worthwhile to follow this view to some extent because, it does bring out the factors involved in considering the effects of ionizing radiation on cells. A considerable amount of work has gone into the study of the effect of ionizing radiation on Escherichia Coli, the microorganism which has contributed so much to our understanding of molecular biology. A most important advance in understanding the action of ionizing radiation on cells came with the observations on HeLa cells in tissue culture by T. T. Puck and P. I. Marcus. If the tissue system is one that requires differentiation in its function and if cells are therefore also reproductively active, then the system will have aspects of sensitivity to ionizing radiation.