This paper reviews current methods of standardizing laboratory investigations in blood clotting, points out their shortcomings, and suggests ways of improving them. Procedures requiring standardization include the diagnosis of haemorrhagic diathesis and thrombotic tendency; the quantification of liver function; and monitoring of the effect of haemostatic replacement therapy and therapeutic anticoagulation. Care in collecting, storing, and handling blood for investigation is of primary importance. Achievements in the standardization of reagents are reviewed and examples of standardized methods are given. Although knowledge of the haemostatic process and experimental methods for its investigation have reached an advanced stage, their clinical application is still unsatisfactory owing to the inadequacy of national and international regulations. To improve the situation, one or more internationally approved laboratories for standardization and quality control, operating according to rules elaborated by experts in the field, should be designated in each country. The need for international reference laboratories is urgent.