Abstract The effect of polyethyleneimine (PEI), a cationic polymer, on protein stability was investigated. Shelf stability of lactate dehydrogenase, stored as an aqueous solution, at 36°C was distinctly improved in the presence of 0.01–1% (w/v) polymer (except at pH 5.0). PEI also prevented aggregation of the protein during long-term storage. Both high and low molecular weight (ca. 600 000 and 2000, respectively) PEI were equally effective. Stabilising effect was also observed for other proteins including alcohol dehydrogenase, β-galactosidase and trypsin. Circular dichroism scan in the far UV range confirmed the protection offered by PEI against complete loss of protein secondary structure during storage. Studies with LDH revealed that the presence of polymer did not increase its denaturation temperature, but on the other hand, hindered the oxidation of free sulfhydryl groups catalysed by metal ions and resultant inactivation of the enzyme. Protection by the polymer was also seen during drying of LDH but not during freeze-thawing.