Objective: Iron fortification of wheat flour is widely used. In most cases, elemental iron powders are utilized as fortificants due to their lower cost and few, if any, sensory problems. However, their bioavailability is unknown. We aimed to measure the bioavailability of H-2-reduced elemental iron powder in white wheat bread made from 72% extraction flour. Design: A stable isotope of H-2-reduced iron powder (mean particle size 15 mum) was used as fortificant in bread prepared from unfortified wheat flour. In all, 12 5- to 7-y-old children were fed bread with 4 mg of H-2-reduced Fe-58/100 g of flour. The next day Fe-57 ascorbate was given as reference dose. After 14 days, erythrocytes were analyzed for isotopic enrichment using mass spectrometry. Results: When normalized to 40% absorption of the reference dose, the geometric mean (+/-range of 1 s.d.) bioavailability of reduced Fe-58 in wheat bread rolls was 6.5% (3.7-11.8). Conclusions: When compared to previous radioiron studies of ferrous sulfate showing 10% absorption from an identical meal in adult women, the relative bioavailability can be estimated at about 65%. However, the bioavailability of this smaller particle size Fe-58 (15 mum) is likely to be higher than that of commercial iron powder (45 mum) although the precise difference cannot be ascertained with current methods. Thus, the bioavailability of commercial elemental iron powders currently used in fortification programs is likely to be substantially lower than that of ferrous sulfate.