Patient 3's IgG was purified on an affinity column of protein A-Sepharose (Pharmacia, Piscataway, New Jersey). The patient's plasma (3 ml) was mixed 1:3 with 0.1 M NaPO 4 (pH 8) and was applied to the column. After the first peak (peak 1) was eluted, the buffer was changed to a low pH buffer (Blo-Rad elution buffer, Richmond, California), and the IgG was eluted (peak 2). These latter fractions were immediately neutralized with 1M Tris/CI (pH 7.0). The peaks were pooled and dialyzed against 0.15 M NaCI, 0.01 M Tris/CI (pH 7.4). Peak 1 had an O D^ = 7.49. Peak 2 was concentrated In a membrane concentrator (Minicon B-15, Millipore, Bedford, Massachusetts) to an O D^ = 6.25. SDS-polyacrylamide gel electrophoresis confirmed the purity of the IgG, with only minor contaminants. Patient and control IgG fractions were also purified by ion-exchange chromatography using a Zetachrome disk (Amicon, Meriden, Connecticut) according to the manufacturer's directions. Fractions were dialyzed against 0.15 M NaCI, 0.01 M Tris/CI (pH 7.4) and then concentrated to an ODaBo of 6.60 (control IgG) and 7.87 (patient IgG) using an immersible filter concentrator (CX-10 ultra filler, Millipore). Patient 3's plasma (2.7 ml) was also chromatographed on a 95 by 1 cm column of Sephacryl S-200 (Pharmacia) at 5 ml/hr using an elution buffer of 0.15 M NaCI, 0.01 M Tris/CI, and 0.05% azide (pH 7.4). Fractions eluted from the column were tested for ODjgg, immunologlc AT-111 content, Immunologic IgG content, and the ability to inhibit the thrombin time of normal plasma. Thrombin times were performed in a semi-automated system (fibrometer, BBL, Cockeysville, Maryland) by warming 0.1 ml of 0.15 M NaCI, 0.01 Tris/CI (pH 7.4), and 0.1 ml of plasma to 37° C for 3 minutes and then adding 0