biological and clinical effects than the same dose delivered in increments at intervals over an extended period of time (i.e., fractionation, protraction). In the latter situation, the interval between exposures allows biological repair of radiation-induced damage to occur so that the total damage is less than if the dose had been delivered acutely, and the clinical effects are correspondingly less severe. The sensitivity of cell systems to radiation is directly related to the rates at which the cells are undergoing division, ie., their mitotic index, and inversely to their level of differentiation. Thus in general, stem cells of rapidly proliferating cell systems are highly radiosensitive (e.g., the stem cells of the hematopoietic systems, spermatocytes). Conversely, muscle and nervous tissue cells are highly radioresistant. Cells having intermediate mitotic rates and differentiation, such as those of the gastric mucosa,fall between these extremes. An important exception to this generalization is the small lymphocyte which, despite its low mitotic index and high degree of differentiation, is highly radiosensitive and an early clinical indicator of acute whole-body irradiation at doses higher than 0.5-1.0 mGy. As hematopoietic stem cells are highly radiosensitive, the extent to which they lie in the radiation field influences the magnitude of the acute whole-body effect of the exposure. Uniform exposure of the whole-body, or a significant portion of it, to penetrating radiation can result in an acute whole-body response that is directly related to dose. If, however, the exposure is non-uniform orlimited to a small area of the body, thereby exposing only a limited variety and numberof stem cells, the whole-body response will be less than if the same dose had been delivered to the whole-body. Theability of internally deposited sources of alpha or beta radiation to induce serious acute whole-body effects (e.g., clinically significant bone marrow depression) is dependent on the amountofactivity and its distribution within the body, and the penetrating power of the emitted radiation. 3.2.2 Early Clinical Features of Acute Radiation Injury Depending on the nature and type of the exposure, exposure to radiation above threshold levels can elicit acute whole-body or local responses that becomeclinically evident within