The development of three-dimensional, enzyme-modified electrodes capable of biochemical transformations at high rate is discussed. Reticulated vitreous carbon coated with poly(aniline) has been used as a substrate for the adsorption bf horseradish peroxidase which was then immobilized with a layer of poly(1, 2-diaminobenzene). More stable modified electrodes resulted if the horseradish peroxidase was also cross-linked with glutaraldehyde. Using the reduction of hydrogen peroxide (2.4 mM) as a model reaction, it is demonstrated that such electrodes operate under partial mass transport control and give a current of similar to 40 mA cm(-3) of 60 ppi reticulated vitreous carbon.