An international collaborative exercise has been undertaken to calibrate a secondary international reference preparation (IRP) of thromboplastin on behalf of the International Committee for Standardization in Haematology (ICSH). This preparation of British Comparative Thromboplastin (BCT/441) is required because supplies of the WHO primary IRP (BCT/253) are necessarily limited. The calibration was performed at seven centres with only a small degree of interlaboratory variation. As a result of this study an ISI value of 1.04 has been assigned to the preparation. Opportunity was also taken to assess the reliability of a simplified calibration based on lyophilised plasmas. The results of the latter appeared reliable. BCT/441 will be available to officially designated National Control Laboratories for calibration of local thromboplastins to promote prothrombin time standardization in oral anticoagulant control.