The binding of [ 1 2 5 I]Hpx-heme with the rat hepatic plasma membrane receptor was studied at 37 o C as well as different parameters such as plasma membrane concentration, calcium dependence, optimal pH and specific binding. A Scatchard plot revealed the existence of one binding for [ 1 2 5 I]Hpx-heme on the isolated liver plasma membrane with a K d = 3.2 x 10 - 8 M.