Purified “terminal” proteolytic E fragments of fibrinogen and fibrin as produced by plasmin digestion were treated with thrombin. Thrombin treatment of fragment E obtained from fibrinogen, liberated both fibrinopeptides A and B and sometimes AP. Thrombin treatment of fragment E obtained from Reptilase-clotted fibrin liberated only fibrinopeptide B. From fragments E derived from thrombin-clotted fibrin as well as from fragments D obtained from either fibrinogen, thrombin-fibrin or Reptilase - fibrin no fibrinopeptides could be recovered.