IONIZING RADIATIONS exert a variety of effects upon biologic material. While many are of great interest and some of exceeding importance, the effect of radiation upon the reproductive integrity of the cell is of paramount importance to the therapist. This is the effect which destroys the ability of cells to undergo multiple mitotic divisions. Although the specific mechanisms by which this is achieved are not fully understood, sufficient knowledge is available to consider modifications of treatment planning to exploit our present knowledge. The remainder of this discussion will be limited to modification of radiation effect upon reproductive integrity. In an attempt to better understand the place and possible mode of action of the various agents that modify radiation effects, a review of a possible sequence of events from ionization to cell death is necessary. These events are diagrammatically depicted in Fig. 1 with the probable major sites of augmentation of effect shown along the schematic diagram. One ...