The spleen and bone marrow have important hematologic functions. Evaluation of bone marrow alone may be incomplete, unless the function of the spleen is studied also. In addition to hematologic functions, each organ has unique characteristics that may need to be evaluated in assessing its activity. The evaluation of the distribution of reticuloendothelial cells, through the phagocytosis of radioactive particles, provides the basis for most of the morphological and functional studies of these organs. During the past three decades, a variety of radioactive agents have been prepared and examined. Among them, colloids of 99mTc, heat-damaged 99mTc labeled RBCs, and the radionuclides of iron and indium, make possible the functional and morphological examination of the spleen and bone marrow by external scintigraphy.