Fractional absorption of an oral dose of iron was determined in calves receiving an irondeficient diet with and without supplementation of iron. Iron supplements were Fe-equivalent doses of oral ferrous sulphate or parenteral gleptoferron. Animals receiving no supplementation showed haematological results suggestive of mild anaemia. Forms of iron supplementation did not differ significantly in their ability to prevent the onset of anaemia. Iron fractional absorption determined using a dual stable isotope labelling method was 60, 39 and 18% in the iron deficient calves, and the calves supplemented with parenteral iron and oral iron, respectively.