The parathyroid, kidney and placenta have been investigated for ultrastructural signs of immunostaining with a murine monoclonal IgG 1-antibody denoted E11. Dispersed cells and tissue sections revealed a finely granular, electron dense precipitate after fixation with periodate-lysine-paraformaldehyde and indirect immunoperoxidase staining with the native or biotinylated antibody. This precipitate was confined to the surface membrane of parathyroid chief cells of normal and adenomatous human glands as well as the bovine and rat parathyroid parenchyma, preferentially the brush border membrane of proximal tubular cells in the human, rat and mouse kidney, cytotrophoblast cells of the human placenta, and trophoblast cells lining fetal blood vessels in the rat placenta. Since the E11 antibody recognizes a large glycoprotein regulating intracellular calcium mobilization and cation fluxes across the cell membrane, the present findings suggest the existence of a similar calcium receptor function on cells involved in different aspects of the calcium homeostasis within a variety of species.