The physical processes by which ionizing radiation interacts with inorganic matter are well known, but radiation interaction with biological materials -- biomolecules, cellular organelles, and organisms -- is inadequately understood. Were we to understand these processes, the use of ionizing radiation in the treatment of disease, specifically cancer, would become considerably more effective. One approach in efforts to gain scientific insights consists of studying the techniques for altering the response of biological systems to radiation damage by the use of radiosensitizers and radioprotectors. To more fully understand the mechanisms by which these materials alter the effects of radiation is to gain a deeper understanding into the chain of events initiated when ionizing radiation is absorbed by biological systems. Specifically, the research proposed in the grant application will pursue the mode of actions of several radiosensitizers (TAN, NEM) and radioprotectors (cysteamine, cysteine) on irradiated systems of B. subtilis and the transforming DNA extracted therefrom. Physical (ESR spectroscopy and analytical ultracentrifugation), biochemical studies (thymine dimers, etc.) and biological studies (bacterial transformation) will all be pursued in preparations under aerobic and anaerobic conditions.