Miniaturized immunosensors, that combine the analytical power of microfluidic devices with the high specificity of biomolecules, have been intensively studied for clinical analyses, environmental immunoassays and biochemical studies. Protein A has the ability to bind Fc-binding site of immunoglobulin G (IgG), and can be used as the detection probe in immunoassays. Ferrocenecarboaldehyde (Fc-CHO) was labeled to protein A as an electrochemical probe for on chip type amperometric flow immunoassay. When labeling Fc-CHO onto protein A via formation of Schiff-base and its reduction, mean number of nine Fc-CHO labeled to individual protein A molecules. Protein A labeled with Fc-CHO exhibited reversible redox properties of ferrocene in cyclic voltammograms and its peak currents increased with number of labeled ferrocenes. The current response of protein A labeled with Fc-CHO well correlated with its concentration when applying 390 V versus Ag/AgCl. Protein A labeled with Fc-CHO showed sufficient electrochemical properties that are useful in a miniaturized electrochemical immunoassays. (C) 2005 Elsevier Ltd. All rights reserved.