1. Iron absorption has been quantitatively measured as the incorporation of physiological doses of stable iron isotopes into erythrocytes. Five milligrams of 57 Fe (orally) and 250 μg of 58 Fe (intravenously) were given to five healthy women on 2 consecutive days. Fourteen days later the changes in the 57 Fe/ 56 Fe and 58 Fe/ 56 Fe ratios in the erythrocytes of each subject were measured using an inductively coupled plasma mass spectrometer. Isotope ratios were also measured in two subjects who were not given any enriched isotope. Concomitant measurements of plasma volume using a dye-dilution technique enabled the estimation of body iron mass and the calculation of iron absorption. 2. The mean coefficients of variation for the 57 Fe/ 56 Fe ratio and the 58 Fe/ 56 Fe ratio were 0.22% and 0.47%, respectively. This precision allowed enrichments of basal ratios to be reliably detected in all cases. The mean change in the 57 Fe/ 56 Fe ratio was 0.00116 ( sd 0.00052, P <0.001) and the mean change in the 58 Fe/ 56 Fe ratio was 0.00035 ( sd 0.00004, P <0.001). Control subjects showed no enrichment. 3. The calculated iron absorption ranged from 10% to 34%, and the amount of absorption was related to the iron stores of the subjects. Percentage iron absorption was identical when estimates of the plasma volume (derived from a body mass equation) were used instead of the plasma volume determined by dye-dilution measurements. Incorporation of intravenous iron into erythrocytes was on average 81% (range 68–93%). 4. The method is especially applicable to the study of iron absorption during pregnancy when incorporation into erythrocytes cannot be predicted.