When tissues are exposed to ionizing radiation, the physical quantity which is employed to specify numerically the degree of irradiation is the absorbed dose. Being the energy absorbed per unit mass of material, this quantity may be used for all radiations. Although such universal applicability has made the concept of absorbed dose very useful in radiobiology, it must be remembered that equal doses of different radiations usually elicit different degrees of biological effect and, what may be even more significant, that inactivation can follow different dose-effect relations. Figure 1 shows the dose-survival curves for immature reproductive cells (spermatogonia) . This figure is based on preliminary data obtained by Bateman and Bond in a co-operative study which we have performed at the Brookhaven National Laboratory (1). The curves reflect the well known fact (2) that these cells are exceedingly radiation-sensitive. However, except for ten times greater radioresistance, curves for mammalian cells in tissue culture are very similar (3, 4). Indeed, it would appear that such survival curves for both low and high linear energy transfer (LET) radiation obtain frequently, and perhaps even regularly, whenever mammalian cells are inactivated. The fact that inactivation by high LET radiation is exponential and subject to little or no recovery, while inactivation by low LET radiation is sigmoid with extensive recovery, has led to the conclusion that the former type of radiation inactivates in single events while the latter does so in multiple events. This difference may be readily appreciated if one considers energy deposition on a microscopic scale as schematically indicated in Figure 2. The photomicrograph on the left side of the figure represents a tubule in the mouse testis. If a mouse is exposed to equal doses of neutrons or γ-rays, the energy absorbed in a region which is about as big as the tubule is very nearly the same. On the other hand, in individual cells considerable differences in energy deposition would be apparent. At doses that are sufficient to inactivate most of the cells there would be few proton tracks in anyone of them in the case of neutron irradiation, but in the case of γ-irradiation several hundred electrons would traverse a cell. If one considers some small volume within the cell, it might be found on the average to be traversed by one proton but still by a considerable number of electrons. Volumes that are still smaller would typically contain no proton track at all, and only rarely would one find a volume traversed by a track representing a very high degree of energy concentration. In such small volumes there would ultimately be found an electron track frequency of one or less.