Male and female blood donors were grouped according to their blood donations, and the iron stores were estimated by a two-site immunoradiometric assay for ferritin. Hb serum iron, serum transferrin and transferrin saturation were also measured. A remarkable low serum ferritin concentration was found in male donors, who had donated blood one or two times. This might indicate that the serum ferritin concentration in male blood donors is not linearly correlated to the iron stores. Among 30 female donors 14 had ferritin values below 10 ng/ml, which have been shown to be indicative of iron deficiency. The serum ferritin concentration could not be used to predict the donors who developed low Hb values by the blood donation which followed. Serum ferritin was correlated to serum iron in men and to serum transferrin and transferrin saturation in both men and women.