An enzyme-linked immunosorbent assay using specific monoclonal antibodies was used to measure circulating transferrin receptor (TR) in 87 patients with various hematologic malignancies. The mean serum TR was significantly elevated in patients with myeloproliferative disorders (15.47 ± 12.54 μg/ml), whereas there were no differences in chronic granulocytic leukemia (7.89 ± 3.56 μg/ml), myelodysplastic disorders (9.25 ± 4.73 μg/ml), and acute nonlymphocytic leukemia (3.85 ± 3.50 μg/ml) as compared to normal (5.63 ± 1.42 μg/ml). Among patients with lymphoproliferative disorders, the mean level was normal in lymphoma (5.73 ± 2.59 μg/ml), multiple myeloma (5.47 ± 1.31 μg/ml), and hairy cell leukemia (7.04 ± 3.69 μg/ml). The serum TR was significantly elevated in chronic lymphocytic leukemia (CLL; 14.17 ± 12.29 μg/ml), and the serum levels reflected the clinical stage of the disease. These findings suggest that serum TR measurement may provide a useful laboratory index of disease activity in certain disorders such as CLL, whereas it most likely reflects the intensity of erythropoiesis in the remaining hematological disorders that were evaluated in this study.