A method for measuring unlabelled Fe absorption has been investigated in a pilot study using a simple mathematical model. The metabolism of newly absorbed Fe can be approximated as a single-compartment model with the sampled compartment being the plasma pool. Five female volunteers (aged 30-55 years) were recruited to participate in the pilot study. After a 10 mg oral dose of unlabelled ferrous sulfate, the change in plasma Fe concentration over the following 6 h was used to estimate the quantity of absorbed Fe from the mathematical model. To assess the accuracy of the new technique, a 1 mg oral dose of (57)Fe-labelled iron sulfate was given simultaneously with a 225 microg intravenous dose of (58)Fe as iron citrate. The plasma appearance of the labelled Fe was used to estimate the absorption of the oral label from the traditional area under the curve method. There was no significant difference (P=0.61) between the geometric mean absorption of the unlabelled (19 (-1 sd 12, +1 sd 28) %) and the labelled Fe (17 (-1 sd 10, +1 sd 29) %). These initial results are encouraging, but further work needs to be undertaken with smaller doses, as typically found in meals. The effect of diurnal variation in serum Fe concentration on the estimation of unlabelled Fe absorption needs further assessment.