A method is disclosed for separating alpha-1-proteinase inhibitor from a blood plasma fraction particularly from the Cohn fractionation scheme. An aqueous solution of the blood plasma fraction is held at a pH of about 6.5-8.5, and a temperature of about 2.degree.-50.degree. C. for a period of about 0.2-24 hours and then mixed with a poly-condensed polyglycol in a proportion of about 10-15 parts of polymer per 100 ml of aqueous solution containing the blood plasma fraction. The mixture may be held at a temperature of about 2.degree.-10.degree. C. for a period of about 1-24 hours. Next, the pH of the mixture is adjusted to about 4.6-5.7, to selectively precipitate unwanted proteins from the solution of alpha-1-proteinase inhibitor which is separated from the solution and purified further.