A new class of fully reversible biosensors, utilizing a layer of monoclonal antibodies trapped at the sensing surface of an immobilized-antigen membrane, is shown to be suited for continuous monitoring of antigen levels. The model sensor described gives a potentiometric response to the hapten 2,4-dinitrophenol (DNP) at micromolar concentration levels through competitive interaction with monoclonal anti-DNP antibodies held within the sensor tip. The proposed design yields a reusable, reagentless antigen sensor with good analytical precision and an operating lifetime of at least 17 days.