The least understood aspect of mammalian iron metabolism is the fate of the metal in the cell cytosol. Transferrin is taken into the cell and held in specific compartments called CURL. The exact mechanism by which the iron is dissociated from the transferrin, the chemical form of iron that enters the cytosol. Furthermore, there is evidence that alternate reductive mechanisms of iron transport are sometimes utilized for cellular uptake, but such processes have been little investigated. A simple model system is lacking. The protozoan, Tetrahymena pyriformis, will be utilized as a model of animal iron transport. This organism, like higher animals, packages ingested iron in vacuoles from which the metal must pass through the cytosol and into mitochondria. A procedure for the rapid purification of these vacuoles will be established. Vacuoles will be isolated containing different forms of iron ranging from microbial siderophores to transferrin. The role of pH, cellular reducing agents, and hydrolytic enzymes in iron solubilization and export into the cytosol will be investigated. The chemical form of iron in the cytosol will be determined and utilized in studying iron uptake mechanisms by the mitochondria. In addition to providing a model for mammalian iron transport, the knowledge obtained may be relevant to the control of infectious diseases caused by protozoa.