The biological effects on mammalian cells of low doses of ionizing radiations will be investigated within the framework of a biophysical theory. A variety of endpoints will be observed including cell lethality, chromosome aberrations, sister chromatid exchanges and oncogenic transformations. The analysis of the data should permit extrapolations of the dependence of cellular effects to doses too low for experimental determination. Modifications of the dose response curves by environmental factors such as oxygen tension as well as chemical and pharmacological agents are an important related objective of the investigation. In addition to intrinsic scientific significance, the results of this research are of pragmatic value in radiation protection and radiotherapy. A variety of radiations are to be applied including neutrons of a wide range of energies, x and gamma rays, alpha radiation and energetic ions up to Z equals 18. The higher organisms irradiated will be principally mammals but insects and plants are to be used in certain studies. The effects under investigation include carcinogenesis, cell transformation, chromosome aberrations and genetic impairment.