Production of manganese-dependent peroxidase (MnP) by Phanerochaete chrysosporium in semi-solid-state cultures is described. The stability of MnP during the production stage and in the crude enzyme preparations was studied. Inactivation of MnP by various factors including proteases, hydrogen peroxide and pH value in semi-solid-state cultures on an inert support (nylon sponge) was studied. Strategies to avoid inactivation are described. Hydrogen peroxide was shown to be essential for enzyme deactivation. A method for the elimination of H2O2 based on catalase addition has been designed which resulted in improvement in both enzyme production and stability.