Method of separating proteins from aqueous solutions

In a method for separation alpha-1-proteninase inhibitor from an aqueous solution containing the same, which comprises the steps of

• (a) holding the aqueous solution containing alpha-1-proteinase inhibitor at a pH of about 6.5-8.5, and a temperature of abut 2.degree.-50.degree. C. for a period of about 0.2-24 hours,
• (b) mixing the solution form step (a) with from about 8% to about 23% (weight/volume) of a polyalkylene glycol, based on the volume of aqueous solution containing the alpha-1-proteinase inhibitor and adjusting the pH of the mixture from about 4.6 to about 7.5 for a time and at a temperature sufficient to selectively precipitate unwanted proteins from the mixture without precipitation of alpha-1-proteinase inhibitor wherein the amount of polyalkylene glycol used may vary 8% to 10% at about pH 4.6 to from about 20% to 23% at pH 7.5 and may be incrementally increased in the range of about 2% to about 3% per 0.5 increase in pH, and
• (c) separating alpha-1-proteinase inhibitor from the solution from step (b),
• the improvement comprising separating alpha-1-proteinase inhibitor from said mixture by salt partition techniques to form a salt phase and a polyalkylene glycol phase by the addition to said mixture of from about 0.5 M to about 1.6 M of at least one water soluble, inorganic salt at a temperature of from about (-)5.degree. C. to about 5.degree. C. and at a pH of from about 5 to about 9 to obtain an aqueous solution containing alpha-1-proteinase inhibitor in said salt phase free of unwanted proteins and substantially free of polyalkylene glycol.