A simple electrochemical immunoassay based on an antibody-modified conducting polymer electrode by direct electrochemical polymerization of pyrrole on a platinum electrode in an aqueous solution and simultaneous doping of the polymer with anti-estrone antibody as the dopant was demonstrated. The specific binding between estrone and anti-estrone antibody immobilized on the electrode surface was evaluated by monitoring the changes in the electrode response of potassium ferrocyanide as the redox marker and a decreased electrode response was observed upon the binding of the estrone to the polypyrrole-antibody (PPy-Ab) modified electrode surface. This change was attributed to the steric hindrance generated from the formation of immunological complex between the electrode surface and the redox marker. A direct and facile method of electrochemical detection of, estrone has been established in this way. (c) 2006 Elsevier B.V. All rights reserved.