any consistent mathematical order in the radiobiological dose relationships. We found that cell survival curves could be very usefully described by a linear- quadratic dose relationship and very soon came to the somewhat premature but, as it turned out, most effective conclusion that the induction of DNA double strand breaks should be linear-quadratic. In deciding that the DNA double strand break was the crucial and all-important lesion we were able to associate the mathematical analysis with the biology of the cell and were able to relate known properties of the DNA molecule to known radiobiological effects. On the other hand, we were restricted and brought, from an abstract two-hit lesion which could have any property one wished, down to earth, to a defined moleccular structure of nanometer dimensions and well-known functions and properties.