Monoclonal anti-parathyroid antibodies have been utilized to isolate a single-chain glycoprotein of 500 kDa, which apparently acts as a sensor of the extracellular calcium concentration and is expressed on the surface of human parathyroid, placental, and kidney tubule cells. The present contribution reports the isolation of a cDNA clone encoding this protein in human placenta and subsequent Northern blots confirming the mRNA expression also in human parathyroid and kidney cells. Close similarity in sequence as well as in tissue distribution is demonstrated with the rat Heymann nephritis antigen, a kidney tubule glycoprotein with calcium-binding ability. The 500-kDa protein belongs to the LDL-receptor superfamily of glycoproteins, claimed to function primarily as protein receptors and characterized by functionally important calcium-binding capacity. It is proposed that the currently identified protein constitutes part of a common structure for the sensing of extracellular calcium concentrations and influences calcium homeostasis in different organs.