Previous methods for purification of C1 inhibitor (C1 INH) have often resulted in a significant loss of the protein's inhibitory capacity. We have employed procedures which permit recovery of this alpha-globulin with an activity equal to that present in normal serum on a per weight basis. Pooled normal human plasma is precipitated with polyethylene glycol (PEG 4000) in the presence of epsilon aminocaproic acid (EACA). The 20 to 40% precipitate contains all the C1 inhibitor antigen. This precipitation results in the exclusion of C1 esterase and plasminogen and represents a 4-fold increase in the concentration of C1 inhibitor relative to plasma. The C1 inhibitor is chromatographed in DEAE cellulose permitting separation from α1 antitrypsin and the majority of albumin. The C1 inhibitor fractions are pooled and run sequentially on Sephacryl-200 and hydroxyapatite columns. The C1 inhibitor obtained after HT chromatography is devoid of contaminating proteins when subjected to disc gel electrophoresis in S.D.S.