The duodenal mucosa of genotypically normal iron replete and iron deficient mice and mice with sex-linked (sla) and microcytic anemias (mk) was examined for the presence of iron-binding proteins. Following continuous, 15 or 120 minute, in vivo intraenteric exposure of a closed duodenal loop to 59Fe, a high speed supernatant of homogenized mucosal tissue was chromatographed on G-200 Sephadex. Two major peaks of 59Fe activity were observed. The molecular weight, and immunological properties of peak I were similar to ferritin whilst those of peak II were similar to transferrin. The distribution of 59Fe between peaks I and II in mk/mk animals was similar to that in genotypically normal iron deficient animals indicating that the intramucosal mechanisms for iron transport were reacting appropriately to the iron deficient state of mice with microcytic anemia. In contrast, the distribution of 59Fe between peaks I and II in sla/Y animals was the reverse of that found in genotypically normal iron deficient animals suggesting the possibility of an intramucosal iron binding protein defect in sex-linked anemia.