Abstract: Preparation of active therapeutic proteins comprising two essential steps. In the first step an active protein source is treated under conditions sufficient to inactivate any viruses present (e.g. via heat or chemical treatment). In the second step, the treated active protein source of moderate purity is then treated under conditions sufficient to remove impurities as well as denatured (once active) proteins resulting from the viral inactivation step. In an illustrative embodiment, a biologically active protein such as antithrombin is treated under conditions sufficient to assure viral inactivation and minimal antithrombin deactivation. After the viral inactivation, the antithrombin is contacted with immobilized heparin to complex only the active antithrombin which is then eluted by known means and processed further (e.g., to include desired excipients and then freeze dried).