Horseradish peroxidase (HRP) has been chemically modified with the homobifunctional cross-linking reagents suberic acid N-hydroxysuccinimide ester (SA-NHS) and ethylene glycol bis-succinimidyl succinate (EG-NHS) yielding derivatives of native HRP with enhanced stability in a range of organic solvents. Modification did not cause any loss of activity. It has been shown that these modification reagents react specifically with the epsilon-amino groups of the six lysine residues of HRP. Stability increased with concentrations of EG-NHS up to a level of 4-5 mg mg(-1). A marked difference was observed in the tryptophan fluorescence profiles of native and the EG-NHS peroxidases upon thermoinactivation at 65 degrees C. Both modified peroxidases exhibited improved resistance to the denaturant guanidine hydrochloride.