The bioavailability in man of commercially available elemental iron powders is unknown despite their extensive use for fortification of flour. Carbonyl iron, which is widely used in Europe, is considered as one of the best reduced iron powders based on studies both in vitro and in animals. In this study, a 55Fe labeled carbonyl iron was prepared by neutron irradiation and used to fortify wheat flour. The native iron of the wheat was extrinsically labeled by 59FeCl3. Doubly labeled wheat rolls were served with different meals. The ratio of absorbed 55Fe/59Fe is a direct measure of the fraction of carbonyl iron that joins the nonheme iron pool and is made potentially available for absorption. This relative bioavailability of carbonyl iron was unexpectedly low and varied from 20 to 5% when the iron fortified wheat rolls were served with different meals. The baking process did not change the relative bioavailability nor the addition of ascorbic acid. The low and variable bioavailability of carbonyl iron in man, makes it necessary to reconsider the rationale of using elemental iron powders for the fortification of foods for human consumption.