The electrical potential between an immunoreactive electrode and a reference electrode in a buffer solution was studied. The immunoelectrode was made of titanium wire, on which an antigen or an antibody was chemically fixed. The electric potential of the electrode sensitized with anti-hCG gamma-globulin shifted in the positive direction in the presence of a small amount of hCG in the solution. On the other hand, the potential of the hCG-sensitized electrode ran in the negative direction upon addition of anti-hCG to the buffer solution. Similar changes in potential were observed between trypsin and its inhibitor, aprotinin. Kinetic analysis was made for the reactions between these species and the change in potential was explained by a simple charge transfer model.