We studied the effect of intravenous (i.v.) administration of 200 mg of iron sucrose following an i.v. bolus injection of recombinant human erythropoietin (r-HuEPO; 300 U/kg body weight) in seven subjects and compared it with seven subjects treated with r-HuEPO alone. Reticulocytes, serum erythropoietin (EPO) and ferritin levels were studied at baseline and daily for the following 8 d. Use of i.v. iron abolished the marked reduction in serum ferritin observed with r-HuEPO administration. Although the total number of reticulocytes was not affected by i.v. iron administration, the reticulocyte Hb content and retHb (a measure in g/l of the Hb contained in all reticulocytes) were increased in the i.v. iron/r-HuEPO group compared with the group who received r-HuEPO alone. Therefore i.v. iron significantly potentiates the haemopoietic response to r-HuEPO in normal subjects.