Abstract An international framework for anticoagulant control has been developed based on British Comparative Thromboplastin (B.C.T.) and the model provided by the British system for anticoagulant control. An alternative international system has been proposed, based on lyophilised thromboplastins prepared at the National Institute of Biological Standards and Control, London. Since 1969, stability studies on two of the N.I.B.S.&C. reagents, the primary material 67/40 and the secondary thromboplastin 69/223, have been in progress at the National (U.K.) Reference Laboratory for Anticoagulant Reagents and Control, Manchester. Both the proposed N.I.B.S.&C. reference preparations have deteriorated, while two lyophilised reagents prepared at the National (U.K.) Reference Laboratory have revealed no evidence of instability. In national and international standardisation reliance on B.C.T. should continue.