Female rats were used to study the loss of plutonium from hepatic and splenic macrophanges. The plutonium was administered intravenously either as a “soluble” [Pu] ferric hydroxide colloid or as an “insoluble” plutonium dioxide suspension. The “soluble” plutonium was lost much more rapidly from the liver than the “insoluble” plutonium. If the iron status of the rats was altered, shortly after the injection of the [Pu] ferric hydroxide colloid, either by the removal of blood or by the injection of colloidal iron, the rate of plutonium loss from the liver was changed. Tile plutonium was retained longer in those animals with excess storage iron than in those with depleted circulatory iron. These results are taken to indicate that iron and plutonium share common pathways of metabolism in macrophages.