A new method is presented for the detection of an immunological reaction in a membrane. Antigens (or antibodies) are taken up in a porous membrane, which covers the gate area of an ISFET. By stepwise changing of the electrolyte concentration, a transient transport of ions through the membrane—protein layer occurs, resulting in a transient membrane potential, which is measured with the ISFET. The transport is determined by the fixed charge density in the protein layer, which changes upon formation of antibody—antigen complexes. No membrane potential is induced at zero fixed charge density. Isoelectric points can therefore be deduced from the registration, and this forms the basis for sensing an immunological reaction. We discuss this new measuring technique and show that antibody concentrations of 10−6 M can easily be detected.