The optimal use of radiation in medicine requires a detailed understanding of the mechanisms determining cellular radiation resistance. Our current understanding is based on a competition model suggesting that cellular radiation damage is initiated through free radicals which decay in one of two major pathways, the first (repair) leading to cell survival and the second (fixation) leading to cell death. The chemical and/or biochemical intracellular environment can be modified to favor either the repair or the fixation of radiation-induced damage, these modifications being based on principles of radiation chemistry. We have made detailed and quantitative tests of the competition model, using cell survival as the assay system, and found various discrepancies between predictions of the model and actual experimental results. These discrepancies could be caused by flaws or oversimplifications of the model. Alternatively, the action of the modifying chemicals could have changed the relationship between early damage (occurring essentially at the same time as the radiation) and final assay (i.e., cell survival, assessed via colony formation after several days of growth). An understanding of chemical modification of damage requires complete knowledge of the radiation-modifying chemicals which exist endogenously. Most previous work has assumed that the dominant endogenous radioprotector is glutathione, but we have recently found that the amino acid cysteine may play an equally important role. Extensive work is planned to quantify this exciting new result, and to determine its general range of applicability. We will test the competition model by comparing whole cell colony formation assays with DNA damage assays. The latter will quantitate several different types of damage. These experiments will include measurements of the uptake and fate of chemical modifiers, but particularly the aminothiol radioprotectors which have been clearly shown not to be in equilibrium between intra- and extra-cellular spaces. The effects of chemical modifiers will be studied under conditions where specific mechanisms of action can be isolated. Other specific goals will be to measure DNA damage under conditions which will maximize our ability to accurately control the concentrations of modifying chemicals. This is often not possible because living cells tend to exclude or concentrate the chemicals of interest. The ability to achieve this goal has recently been established for the study of DNA damage in an isolated nuclei model. This model system will allow the measurement of DNA damage with a degree of experimental control previously obtained only for chemical systems, yet with structural and biochemical aspects of DNA relevant to its natural state in higher organisms.