Ferritin is the major iron-storage protein in vertebrates, plants, fungi and bacteria. The term ferritin goes back to the early observations of O SCHMIEDEBERG (1894) who was one of the first to report on iron containing proteins in animal tissues. Due to its high iron content (average iron content 23%) and electron scattering density, ferritin molecules can be directly observed in the electron microscope. For immunohistological purposes at the electron microscopic level, ferritin from horse spleen is preferred and usually conjugated with antibodies according to procedures described by SJ SINGER (1959) or RA RIFKIND et al. (1960).