The duodenal mucosa, primary site of iron absorption, is by means of some mechanism informed about the body's requirement for iron. It acts upon this information, absorbing more iron when the requirement is greater, less when requirement is normal. It is not known how the information is transmitted to the intestine nor how the absorptive cells translate the information into appropriate behavior. We propose to examine three channels through which information concerning iron requirement might be transmitted to the duodenum. (1) Plasma transferrin saturation (2) Plasma ferritin (3) Monocyte-macrophage. The participation in iron metabolism of each of these remains partially unexplored, and we shall continue to study the ways in which ferritin, transferrin and the monocyte-macrophage serve as transport mechanisms for iron in both plasma and tissue. Recognizing hemochromatosis as a disorder of the control of iron absorption we intend this research to improve our understanding of the pathogenesis of that syndrome.