Monoclonal antibody reagents were used to develop a sensitive enzyme-linked immunoassay for clinical measurement of circulating transferrin receptor. By using transferrin-bound receptor for the preparation of the immunologic reagents, we developed an assay that gives an identical dose-response curve with either free or transferrin-bound receptor. The mean concentration of circulating receptor in 82 normal male and female volunteers was 5.63 +/- 1.42 mg/L. The level was reduced significantly in patients with primary aplastic anemia and post-transplant aplasia (2.58 +/- 1.07 mg/L and 2.32 +/- 0.48 mg/L, respectively) and was sharply elevated in patients with hemolytic anemia and iron deficiency anemia (33.1 +/- 17 and 18.0 +/- 11.4 mg/L, respectively). Our assay values are approximately 20-fold higher than results published previously in a study that used an immunoradiometric assay. The disparity apparently relates to a difference in sensitivity of the latter assay for free and transferrin-bound receptor. Measurements of serum transferrin receptor provide a useful clinical index of either total or iron-deficiency erythropoiesis.