This volume contains all relevant information discussed in a Workshop on thromboplastin calibration held in Leiden, The Netherlands on July 1, 1983. The Workshop was an initiative of the Dutch foundation for a Reference Laboratory for Anticoagulant Control (RELAC) and it was organized by the Boerhaave Committee for postgraduate teaching of the Faculty of Medicine of the University of Leiden. The Workshop was held under the auspices of five organizations i. e. the European Community Bureau of Refer ence (BCR), the European Committee for Clinical Laboratory Standards (ECCLS), the International Association of Biological Standardization (lABS), the International Committee for Standar dization in Haematology (ICSH), ·and the International Committee on Thrombosis and Haemostasis (ICTH). The aim of the Workshop was to discuss and develop a method for calibration of reagents, i. e. thromboplastins and/or plasmas used for the prothrombin time. During the Workshop three recent thromboplastin calibration studies were discussed, the results of which are presented in chapters 4, 7 and 9. These studies were carried out on the basis of a new calibration model developed by experts working with BCR and WHO. The usefulness of this model and the standardization system based on it is the leading thread running through this volume. Statisticians and clinicians discuss the results from a scientific point of view. Thromboplastin manufacturers, for whom especially the use of the model and the system is intended, discuss the matter also from an economic and legal point of view.