Absorption of iron was studied with a double-isotope technique that allowed differentiation between “mucosal uptake,” “mucosal transfer,” and ultimate “retention” of iron. A physiologic dose of ferrous sulfate was administered to 25 healthy young adults, 40 active aged persons, and 20 patients with uncomplicated iron deficiency. Radioactivity was measured with a whole-body scanner. Iron absorption values were not decreased in aged subjects compared to young adults. Mucosal uptake, mucosal transfer, and retention of iron were equally increased in both young and old patients with iron deficiency. In 12 young adults and 33 aged persons red cell iron uptake was studied in addition to iron absorption. Young adults utilized 91% of the retained, orally administered iron and the aged only 66%. An increase in ineffective erythropoiesis in old age is suggested.