1. It has been shown that powdered hematin is exceedingly resistant to the metabolic action of tissue cells, and wherever injected it will remain for weeks with but slight and slowly progressing alteration. 2. The changes produced in hematin by tissue cells are identical with those changes resulting from the oxidation of hematin by hydrogen peroxid, forming a series of bodies of decreasing color intensity, which manifest an iron reaction in inverse proportion to their color. 3. These hemosideroid pigments are distinguished from true hemosiderin by the type of their iron reaction, by their solubility in dilute alkalies, and by their destructive oxidation by hydrogen peroxid. 4. Crystalline hematin injected into the tissues is subject to the same changes as powdered hematin. 3. Whereas powdered parahemosiderin is converted into hemosiderin by the tissue cells with comparative ease, powdered hematin shows no such conversion within forty-seven days, and as the only demonstrable change produced in hematin by the tissue cells does not result in the formation of either hemosiderin or hematoidin, the assumption that hematin is the progenitor of these pigments seems unwarranted.