AbstractPurified antithrombin III (AT III), a single‐chain human plasma glycoprotein, molecular weight 58,000 daltons, and one of the major serine protease inhibitors, was heated in the 60–70°C range for inactivating possible contaminations by hepatitis B virus (HBV). Loss of inhibitory activity, unfolding of tertiary structure, and the rate of aggregate formation of AT III were monitored experimentally during heatig. Sucrose and sodium citrate were demonstrated to stabilize the protein. From the rate data the calculated activation energies (E) showed Etert. struct. < Ebiol. act. < Eaggreg. indicating the order (lower activation energy process first) in which heat causes these changes in the protein molecule. The activation energy corresponding to denaturation of HBV was estimated to be at least fourfold lower than that associated with the unfolding of the tertiary structure of the protein. Purified AT III, thus stabilized and pasteurized, should be therapeutically effective, and the risk for transmission of hepatitis B should be decreased significantly.