several procedures are known to modify the response of mammals to ionizing radiation. Prominent among these are methods and agents that prevent damage to the hematopoietic system and overcome endogenous bacterial invasion. It is significant that the reticuloendothelial system (RES), which shows no evidence of damage histologically even after exposures of 2,000 R or more, constitutes a major defense mechanism of the host against bacterial agents (1) and an important source of regenerating hematopoietic cells (2). Exhaustion of certain cells of RE origin has been proposed as the prime cause of death after whole-body irradiation (3). Evidence has been accumulated that implicates the participation of the RES in ho-meostatic adjustments to severe stresses and indicates that this system plays an important role in radiation recovery. Materials and Methods Young adult rats of the CF-Nelson strain (140–175 g) were used in these experiments. Whole-body irradiation was administered by a therapeutic unit operated at ...