The development of reagentless biosensors is of fundamental importance for the improvement of the sensor characteristics. In this respect, direct electrical communication between the enzyme and an electrode surface has to be established. We could demonstrate that in the case of biocatalysts catalyzing the same redox process (H/sub 2/O/sub 2/ reduction) e.g. cytochrome c, microperoxidase MP-11 and horseradish peroxidase their specific catalytic activity plays only a minor role for their function in an amperometric enzyme electrode. By far more important is the distance between the active site of the enzyme and the electrode surface. To achieve the smallest possible distance it is advantageous to use the biocatalyst with the best access to its active site and the smallest molecular weight. Using microperoxidase MP-11 instead of horseradish peroxidase the electrocatalytic reduction of H/sub 2/O/sub 2/ could be increased by a factor of about 18,000 as compared with the specific enzymatic activity in solution due to the direct electron transfer between the monolayer-immobilized minizyme (minimized enzyme) and the electrode surface.