An amperometric immunosensor for the detection of the herbicide atrazine has been developed. A redox polymer PVPOs(bpy)(2)Cl was co-immobilized with the specific antibody on the surface of the electrode by crosslinking with PEGDGE to form an electron-conducting hydrogel, In a competitive assay the occurrence of the antibody-antigen reaction on the surface of the sensing film was detected through the 'electrical wiring' of the redox centres of antigen-labelled horseradish peroxidase and the electrode surface in the presence of H2O2 at 0.1 V (vsAg/AgCl). (C) 1998 John Whey & Sons, Ltd.