It is now clear that the initial geometrical distribution of primary radiation products in irradiated biological matter is fundamental to the observed end-point (cell killing, mutation induction, chromosome aberrations, etc.) In recent years much evidence has accumulated, indicating that, for all radiations, physical quantities averaged over cellular dimensions (micrometers) are not good predictors of biological effect, and that energy deposition processes at the nanometer level are critical. Thus irradiation of cells with soft x rays whose secondary electrons have ranges of the order of nanometers is a unique tool for investigating different models for predicting the biological effects of radiation. We demonstrate techniques whereby the biological response of the cell, and the physical details of the energy deposition processes may be separated or factorized, so that given the response of a cellular system to, say soft x rays, the response of the cell to any other radiation may be predicted. The special advantages of soft x rays for eliciting this information and also information concerning the geometry of the radiation sensitive structures within the cell are discussed.