Polyethylenimine (PEI) interferes strongly in all commonly employed protein estimation assays. Here, a strategy to remove this interference is developed. Polyethylenimine is selectively removed by adsorption from a mixture containing PEI and lysozyme using carboxymethyl-cellulose, a cation-exchange resin between an ionic strength of 0.35-1 M (pH 6.0), conditions under which lysozyme remains in solution and can be accurately estimated.