CLINICIANS HAVE long desired safe and dependable radiographic visualization of the liver and spleen. One approach to this problem is based on the ability of the reticulo-endothelial cells of these organs to phagocytize colloidal particles from the blood flowing through their finer vascular channels. Selection of a colloidal substance of a relatively high atomic number is essential to provide contrast with adjacent tissue. If, then, the liver and spleen are perfused over a limited period with a large number of minute particles of high atomic number, the constituent cells will engulf the particles, each such cell becoming more radiopaque than cells not containing these particles. As a result, the entire organ may be demonstrated by standard radiographic technics. Also essential to success is a particle of proper size, one which will not occlude a capillary. When the contrast agent is susceptible to metabolic change or excretion, a time limit for its introduction and for performing the radiographic study enters the problem. To be safe and trustworthy, the method must be without immediate or eventual harm to local tissue or the whole organism. A low incidence of minor reactions may be tolerated, but serious acute reactions to a procedure prohibit its use. Ample evidence must also exist to insure against any subacute or chronic disability stemming from the use or retention of the medium. In previous attempts at hepatolienography based on the phagocytic function of liver and spleen cells, heavy metal compounds, the oxides of thorium and tantalum (1-3), and iodinated fatty substances (4-7) have been employed. Their injection, however, has produced either acute reactions or delayed injury to cells and organs. Ethyl triiodostearate and ethyldiiodostearate have not been widely accepted because of frequent minor and severe reactions attending their intravenous injection (5, 8). Thorium dioxide has been condemned because of its radioactivity and its permanent retention in the body (9). Some evidence has indicated that it is productive of tissue fibrosis and stimulates neoplasia. Colloidal stannic oxide, a new hepatolienographic agent, shows considerable promise of being safe and dependable (10). Preliminary animal studies indicate that it can be given intravenously without acute or delayed reaction. Tin, of atomic number 50, is of sufficient radiopacity that excessive amounts are not necessary to obtain dependable radiographs. Being insoluble, and in colloidal form, the oxide is selectively deposited in the reticulo-endothelial cells, with only a negligible portion remaining in other body tissues. It is not radioactive. While it is indefinitely retained in the body, ample evidence exists to show that it is innocuous. Elimination of stannic oxide from the body is apparently not possible on the basis of present evidence.