Cutter Biological, Miles Inc., has implemented large-scale isolation and purification of antithrombin III (ATIII) from human blood plasma. The starting material, Cohn fraction IV-1, is suspended and ATIII separated from other proteins by heparin-affinity chromatography. The concentrate is subsequently heated for 10 hours at 60 degrees C in a solution of 0.5 M sodium citrate to inactivate the causative agents of viral hepatitis B and acquired immune deficiency syndrome. After heating, a second heparin-affinity chromatography step is employed to isolate the active ATIII and to remove heat-denatured protein. The purified solution is compounded with 0.1 M alanine and 0.15 M sodium chloride, filled in vials of 500 or 1,000 IU each, and freeze dried. The final product has a specific activity of not less than 6.4 IU ATIII per mg protein, of which over 90 percent binds to heparin on crossed immunoelectrophoresis. Preclinical and clinical testing have shown this product to be safe for prophylactic and therapeutic administration to persons with congenital deficiency of ATIII.