To the internist and the pathologist siderosis means the deposition of pigment from the blood in various organs of the body, such as the spleen, liver, lungs, bone marrow and lymph nodes. Of course, the presence of free iron in any large amount in the blood stream would result in the deposit of this iron in the aforementioned organs also, as Migay and Petroff showed when they injected iron and also carmine blue into the blood streams of dogs. The spleen and liver contain the greatest amounts of iron. It is not present in the tissue cells of these organs, but is held in the reticulo-endothelial cells, which are specially differentiated phagocytic cells. The lymph nodes and the bone marrow contain a moderate amount of the foreign particulate material in these reticulo-endothelial cells, but the lungs contain much more, and the spleen and liver the most ; there is none in