Recent studies have provided immunological evidence for the existence of transferrin receptor in human serum and have revealed that its concentration is a sensitive measure of erythropoiesis and iron deficiency. The present study was undertaken to establish the molecular identity of this immunoreactive component. Purification from human serum was accomplished by immunoaffinity chromatography using, as the ligand, monoclonal antitransferrin receptor antibody. The receptor preparation contained two major components with Mr of 75,000 and 85,000, which were identified as transferrin and transferrin receptor, respectively. The physicochemical and immunochemical properties of the 85,000 serum receptor were compared with those established for intact placental transferrin receptor. The serum receptor exhibited an apparent Mr = 85,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis under non-reducing conditions, as compared with 190,000 for placental transferrin receptor. Upon reduction, the Mr of serum receptor was unaltered, whereas, the 190,000 placental receptor dimer decreased to the expected monomer value of 95,000. Amino-terminal amino acid sequence analysis revealed that residues 1-19 of serum receptor were identical to residues 101-119 of intact receptor. These findings provide physicochemical evidence for the existence of transferrin receptor in human serum, establish its molecular identity as a truncated form lacking the cytoplasmic and transmembrane domains (residues 1-100) of intact receptor, and demonstrate that it exists as a transferrin-receptor complex in serum.