The shifts in membrane potential, caused by the injection of hormones into a permeation cell, were measured using immobilized (entrapped) serum albumin and gamma-globulin membranes. The effective fixed charge density was estimated to increase after the injection of estradiol and testosterone in both albumin and gamma-globulin membranes, while the charge density was estimated to decrease after the injection of progesterone in the gamma-globulin membranes. Because the change in the charge density originates from the conformational change of proteins in the membranes, the change in the circular dichroism induced by the hormones was measured in the membranes. The alpha-helix content in both albumin and gamma-globulin membranes was found from the circular dichroism measurements to increase when estradiol and testosterone was bound to the proteins, while the alpha-helix content in the albumin membrane decreased on the binding of progesterone. Some discrepancy was found between the conformational change of the proteins in the membranes detected by the membrane potential measurements and the circular dichroism measurements. This is explained by the fact that the circular dichroism measurements do not directly contribute to the change in the charge density induced by the binding of hormones to proteins in the membranes.