A mixed-monolayer of an antigen and electrocatalyst assembled onto Au surfaces acts as an immunosensor electrode for the amplified amperometric detection of the complementary antibody (Ab). In the presence of a substrate for the electrocatalyst, the mixed monolayer electrode yields an electrocatalytic current which is blocked by association of the Ab to the monolayer. A mixed monolayer of N epsilon-2,4-dinitrophenyl lysine and microperoxidase-11 assembled on an Au electrode is applied to the amperometric detection of the dinitrophenyl antibody (DNP-Ab) in the presence of persulfate, S2O82-, acting as substrate for the microperoxidase electrocatalyst. Similarly, a mixed-monolayer of N epsilon-2,4-dinitrophenyl lysine and pyrroloquinoline quinone (PQQ) assembled on an Au surface acts as an immunosensing electrode for the DNP-Ab in the presence of 1,4-dihydronicotinamide adenine dinucleotide (NADH) and Ca2+ ions. The electrocatalytic oxidation of NADH in the presence of Ca2+ ions by the PQQ electrocatalyst, and the inhibition of the electrocatalyzed process upon association of the DNP-Ab to the electrode, are used in the analytical sensing of the DNP-Ab. The electrode is sensitive for determination of the DNP-Ab in the concentration range 0.2-20 mu g ml(-1).