Fe absorption was estimated in adults and infants from the erythrocyte incorporation of Fe isotopes added to infant formula. Fe absorption was measured in adults using radioisotopes, and in infants with a stable-isotope technique. In adults, the geometric mean Fe absorption from a ready-to-feed soya formula with its native phytic acid content was 2.4%. This increased to 6.0% (P < 0.05) after almost complete dephytinization. In infants, mean Fe absorption values were 3.9 and 8.7% respectively from the same products (P < 0.05). In adults, mean Fe absorption from a spray-dried soya formula containing 110 mg ascorbic acid/l was 4.1%, increasing to 5.3% (P < 0.05) when ascorbic acid was doubled to 220 mg/l. In infants, mean Fe absorption values were 5.7 and 9.5% (P < 0.05) from the same products. Mean Fe absorption from a milk-based formula was 6.5% in adults compared with 6.7% in infants. All meals in the adult and infant studies were fed using an identical meal size of 217 g. Increasing the meal size threefold in adults did not change fractional Fe absorption. Mean Fe absorption values for each meal were lower in adults than in infants but the relative inhibitory effect of phytic acid and the enhancing effect of ascorbic acid were similar. We conclude that Fe absorption studies in adults can be used to assess the influence of enhancers and inhibitors of Fe absorption in infant formulas fed to infants. Further studies, however, are required to extend these findings to weaning foods and complete meals.