1. The isotope technic for studying iron absorption has been extended to measure the unabsorbed isotope in feces as well as the amount synthesized into hemoglobin. The recovery of radioiron from feces was shown to be accurate within 10 per cent.
2. There was suggestive evidence to indicate that, with the 1 mg. per kilogram dose employed, normal subjects may sometimes absorb more iron than is converted within a two week period into hemoglobin.
3. Patients with fever, untreated pernicious anemia, and refractory anemia were shown to absorb more iron than they use for hemoglobin.
4. Patients with hemolytic anemia may absorb more iron than can be recovered in the peripheral blood at any one time because isotopic hemoglobin is removed from the circulation at a rapid rate.
5. Except in afebrile patients with hypochromic anemia, acceptance of the per cent of a given dose of radioiron which appears in circulating hemoglobin as a measure of iron absorption must be made with caution.
6. The theory that mucosal cells accept iron for absorption or block its assimilation provides the best known explanation for iron absorption; but patients with adequate iron stores may assimilate considerable quantities of the metal and the block must be regarded as relative.