The rate of clearance of colloidal carbon was measured in mice up to 7 days after whole-body irradiation with supralethal doses of X-rays and up to 8 weeks after exposure to mid-lethal or sublethal doses. The clearance rate was increased up to 2-fold in irradiated animals and there was a clear dose-response relationship. The effect was also markedly dependent on the length of time after irradiation. It was concluded on the basis of organ uptake studies and relative numbers of carbon containing cells in liver sections that the increased rate of carbon clearance is the result of a greater phagocytic capacity of liver macrophages. A proportion of mice irradiated with 800 rads developed symptoms of “secondary disease” 6 weeks after the exposure which were accompanied by an increased phagocytic function of the RES.
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