A method is described which enables simple and accurate measurement of the separate steps of iron absorption. A double isotope technique was used with 59Fe as a tracer and 51Cr (as CrCl3) as an inert, not absorbable indicator. With a whole-body counter it was possible not only to measure initial mucosal uptake and retention but also to calculate mucosal transfer of iron. Thus faeces collection for determination of mucosal uptake of iron was no longer necessary. Absorption of a physiological dose of inorganic iron was studied in 25 normal subjects (15 males and 10 females), in 21 patients with uncomplicated iron deficiency and in 8 patients with idiopathic haemosiderosis. A significant difference was demonstrated between normals and patients with iron deficiency and in idiopathic haemosiderosis with respect to the various steps of iron absorption. The method provides a simple tool for the study of the regulation of iron absorption at the luminal and intracellular site of the intestinal mucosal cell.