The shifts in membrane potential, caused by the injection of some amino acids into a permeation cell, were measured using immobilized gamma-globulin membranes. The shifts in membrane potential were observed to be positive or negative when the isoelectric point of each amino acid injected into the cell was less or higher than 6.0. The potential response caused by the injection of each amino acid shows an individual and characteristic curve depending on the amino acid, and the difference in potential curves between D-aspartic acid and L-aspartic acid is significantly observed in the immobilized gamma-globulin membranes. The t3/4 value was found to increase in the following order : lysine = glutamic acid < arginine < D-aspartic acid = asparagine < L-aspartic acid < histidine < alanine, where t3/4 indicates the time at which 75% of the shifts in membrane potential has been observed. The modified membrane potential theory provides satisfactory explanations for the membrane potential obtained experimentally before and after the injection of L-alanine, and the theoretical shifts can explain the experimental shifts in membrane potential due to the injection of L-alanine into the cell.