The direct and mediated mechanisms of the electroreduction of hydrogen peroxide at a graphite electrode modified with horseradish peroxidase (HRP) were studied. The turnover number of the heterogeneous electron transfer between adsorbed HRP and the electrode was found to be equal to 0.66 +/- 0.28 s(-1). The rate of the reaction of H2O2 with HRP on the graphite surface was found to be 385 times slower than that in solution. p-Cresol, phenol and p-chlorophenol behaved as efficient mediators in the process of bioelectrochemical H2O2 reduction. From the comparison of kinetically limited currents observed during direct and mediated reduction of H2O2 it was concluded that the population of adsorbed HRP molecules and/or the graphite surface structure cannot be treated as homogeneous. It was found that 42% of the total amount of HRP molecules adsorbed on the electrode were accessible for direct unmediated electron transfer from the graphite electrode.