The absorption of iron from slow-release and rapidly-disintegrating ferrous sulphate tablets has been compared using a double radioiron isotope technique. The studies were performed in 15 normal subjects, 20 blood donors and 10 patients with iron deficiency anaemia. The preparations containing 100 mg of ferrous iron were given twice daily on alternate days for 10 days. The absorption differed significantly between the three groups of subjects, being highest in the anaemic patients and lowest in the normal subjects. In all groups, significantly more iron was absorbed from the slow-release tablets compared to the rapidly-disintegrating tablets. The mean absorption ratios were 1.3 in both normal subjects and blood donors. In patients with iron deficiency anaemia the ratio was 1.4.