One hour after intraperitoneal injection of iron-dextran or saccharated iron oxide into mice, characteristic deposits were present in splenic macrophages, in sinusoidal endothelial cells of spleen and liver, and in vascular endothelial cells of various renal capillaries. Four hours after injection, small numbers of ferritin molecules were identifiable about intracellular aggregates of injected iron compounds; and by the 6th day, ferritin was abundant in close proximity to deposits of injected iron compounds. The latter were frequently situated in cytoplasmic vesicles delimited by single membranes. These vesicles were most frequently found in tissue obtained during the first 6 days after injection; and in certain of the vesicles ferritin molecules surrounded closely packed aggregates of injected material. Much unchanged ferric hydroxide was still present in macrophages and vascular endothelial cells 3 to 4 weeks after injection.

While electron microscopy left no doubt about the identity of injected ferric hydroxide on the one hand, and of ferritin or hemosiderin on the other, histochemical tests for iron failed in this respect.

Precipitation of ferric hydroxide (hydrous ferric oxide) from stabilized colloidal dispersions of iron-dextran was brought about in vitro by incubation with minced mouse tissue (e.g. liver), but not by incubation with mouse serum or blood.

Subcutaneous injections of hydrous gel of ferric oxide into mice initially produced localized extracellular precipitates. Most of the material was still extracellular 16 days after injection, though part of it was phagocytized by macrophages near the site of injection; but apparently none reached the spleen in unaltered form. Five days after injection and thereafter, much ferritin was present in macrophages about the site of injection and in the spleen.

The findings show that iron preparations widely used in therapy can be identified within cells, and that their intracellular disposition and fate can be followed at the molecular level. Considered in the light of previous work, they indicate that the characteristic structure of the ferric hydroxide micelles in molecules of ferritin is specific, and develops during the union of apoferritin with ferric hydroxide. Apparently this union does not depend upon specific cell components.