Abstract: There is disclosed an improved method for separating one of alpha-1-proteinase inhibitor (also known as alpha-1 antitrypsin) and antithrombin-III from an aqueous solution of plasma proteins containing the same, such as from Cohn Fraction IV-1, Cohn Fraction IV, reworks of Cohn Fraction IV and IV-1, Cohn Effluent II & III and Cohn Effluent I. The method includes the steps of first holding an aqueous solution of plasma proteins containing one of alpha-1-proteinase inhibitor and antithrombin-III in a relatively large volume of buffer solution as solvent and at a pH adjusted to be relatively basic when compared to conditions heretofore known, and at a temperature in the range of from 2.degree.-50.degree. C. for a period of about 0.2-24 hours. Following the above treatment, alpha-1-proteinase inhibitor and antithrombin-III are obtained by applying conventional techniques to the resulting solution. Accordingly, the solution is then mixed with a polyalkylene glycol, e.g.