concentration of about 0 8 and the mixture is allowed to settle for an appropriate period generally in the range of from about 30 to about 90 minutes the mixture is then centrifuged and the supernatant collected excellent results are obtained when the polyethyleneimine is added as a 10 v v solution in h 20 in an amount sufficient to result in the polyethyleneimine consisting of from about 0 7 to about 0 8 v v of the total solution the ph of the solution is 8 u00b1 0 5 preferably u00b1 0 1 and the temperature is held in the range of from about 2 u00b0 to about 15 u00b0 c the protein concentration in the supernanant is determined at this stage and at each further processing stage by the standard coomassie blue binding assay another procedure for removal of the nucleic acid is by using chromatography on hydroxyapatite or immobilized pei precipitation with protamine sulfate is another useful procedure after removal of the nucleic acids the gamma interferon containing mixture is subjected to a first protease removal step the most convenient method for removing the proteases is by chromatography of the supernatant from the nucleic acid removal step utilizing an anion exchange resin quarternary aminoethyl mixed amine or other intermediate base resin or a weak base resin such as p amino benzyl cellulose are particularly useful quaternary aminoethyl is a preferred anion exchange resin the quarternary aminoethyl.