The feasibility of an immunosensor on the basis of impedance analysis has been studied. The measurements were performed in a classical three-electrode electrochemical cell. As a sensitive element a Si/SiO2 heterostructure was used grafted with anti-alpha-fetoprotein antibodies through three different coupling reagents : aminosilane, cyanosilane and polysiloxane membranes. Promising results have been obtained in such a system with polysiloxane membranes (after antigen-containing analyte incubation) through an analysis of impedance variations. The dynamic range of this sensor is between 10 and 150 ng ml(-1). The results presented show good selectivity, reproducibility, and storage stability for the immunosensor, at least during 50 days of storage in PBS at +4 degrees C.