Elastase from Pseudomonas aeruginosa has recently been used successfully for peptide synthesis. To improve its performance we attempted to increase its catalytic stability by chemical modification. Two distinct sorts of amino group-specific modifiers, dimethyl suberimidate and cyanuric chloride-activated polyethylene glycol (PEG), gave a two-fold increase in catalytic stability at 70 degrees C and greater degrees of stabilization at lower temperatures. Suberimidate treatment seemed to act by intramolecular crosslinking, whereas the activated PEG gave rise to an elastase-PEG conjugate. The thermal transition (T-m) for suberimidate-treated elastase was unchanged from the native value of 72 degrees C. PEG-conjugated elastase gave anomalous T-m curves; therefore, a value could not be determined. The lack of correspondence of catalytic stabilization with increased T-m values is discussed.