The kinetics of the direct electron transfer from a graphite coating as an electrode to compounds I and II of adsorbed horseradish peroxidase (HRP) is analyzed. The surface density of HRP on the graphite coating measured by using a quartz crystal microbalance is 1.4 x 10(-11) mol cm(-2). The apparent heterogeneous electron transfer rate constant is 2 x 10(-4) s(-1) at pH 6.4, which is three orders of magnitude smaller than that for HRP adsorbed on a normal graphite electrode.