Thermal stability of horseradish peroxidase (HRP) has been enhanced by acetylation with acetic acid N-hydroxysuccinimide ester (AA-NHS) under mild conditions. The half-life at 65 degrees C was increased fivefold. This modification has also resulted in greater tolerance of the water-miscible organic solvents, dimethylformamide and tetrahydrofuran, at 25 degrees C and 60 degrees C and of methanol at 60 degrees C. Analysis of the free amino groups of HRP indicates that AA-NHS alters 3 of the HRP 6 lysine residues. This stabilization has been achieved by a simple chemical modification involving neither immobilization nor the use of cross-linking agents.