A method for large-scale production of a pasteurized antithrombin III (AT III) concentrate for therapeutic use has been adapted from published methods. It includes the following steps: (1) batchwise adsorption onto heparin-Sepharose from plasma depleted of cryoprecipitate and prothrombin complex; (2) chromatographic elution at high salt concentration; (3) pasteurization for 10 h at 60°C in the presence of added citrate ion; (4) desalting on Sephadex G-25, and (5) sterile filtration and freeze-drying. Seven batches prepared in this manner gave a mean yield of 269 U AT III/kg plasma. The product passed all the usual animal safety and pyrogenicity tests and has been used successfully in several courses of treatment of congenital deficiencies.