Localization of intracellular ferritin in leukemic cells and normal leucocytes from 31 patients with leukemia has been studied by light and electron microscopic immunoperoxidase procedures. With light microscopic observation, positive immunostaining for ferritin was more frequently observed in leukemic cells under conditions of myelogenous and monocytic leukemia. It is noted that positive leukemic cells tended to increase in number in proportion as higher serum ferritin levels in myelogenous leukemia. On the other hand, a majority of leukemic cells and normal leucocytes in lymphatic leukemia exhibited weak or negative immunostaining for ferritin, even in cases with higher serum ferritin levels. With electron microscopic observation, intracellular ferritin which was detected as positive immunostaining with deposits of DAB molecules was diffusely distributed only in cytosol of any positive leukemic cells and normal leucocytes. There was no immunostaining for ferritin in nucleus, rough endoplasmic reticulum, Golgi complexes, mitochondria, and intracytoplasmic granules. Immature and blastic leukemic cells which contained intense positive immunostaining for ferritin were observed in acute myelogenous leukemia and blastic crisis of chronic myelogenous leukemia with considerably high levels of serum ferritin. These results suggested that myelogenous leukemic cells synthesized increased amounts of ferritin by free ribosomes and contributed to high levels of serum ferritin.