The relative intestinal absorption of heme- and non heme-iron in connection with a standardized test meal was studied in a group of fertile women given 16 mg Fe in the form of FeSO4 and 2 mg Fe in the form of hemoglobin. Both in normal subjects and in women with iron deficiency, the heme-iron was significantly better absorbed (16.13% +/- S.D. 8.0 vs 4.59 +/- 3.4, p < 0.01 and 22.03 +/- 8.9 vs 9.45 +/- 7.8, p < 0.05). For targeted prophylaxis of iron deficiency with small, side-effect-free doses, heme-iron is thus a valuable component which increases the absorption by about 40%. Heme-iron does not cause high concentrations in the intestinal lumen of free radical inducing, possibly harmful ferric iron.