The activation of factor IX purified from human plasma has been studied. Factor XIa and kallikrein separately activated factor IX to factor IXa. In both cases factor IXa had an apparent molecular wight of about 42-45000 in sodium dodecyl sulphate-polyacrylamide disc gel electrophoresis compared with a molecular weight of about 70000 for the native factor IX. The activation by XIa required Ca2+-ions, wherease Ca2+-in and factor VII or Russell's-viper venom alone did not activate factor IX. Trypsin activated and plasmin inactivated factor IX.