The Department of Laboratory Medicine, Seoul National University Hospital has had an opportunisity to participate in "The International study group for anticoagulant control" which has been organized by "National(U.K.) Reference Laboratory for Anticoagulant Reagent and Control" for 2 years since 1980. Four trials have been performed a year for Prothrombin time, Partial thromboplastin time, Factor VIII assay, Fibrinogen, Antithrombin III and Thrombin time in various combinations. Over 345 laboratories in England and over 80 laboratories from overseas have participated in this program. Based on the cumulative experiences with this international quality control for coagulation, the following experiences were obtained. 1) Except for a few test items of some trials, our laboratory obtained relatively satisfactory results. 2) Large variety of laboratory techniques and reagents were used in each laboratory. 3) Percent deviation was used to estimate the reliablity of interlaboratory results in prothrombin time in second and prothrombin ratios. 4) Many laboratories interpreted the results of prothrombin activities(%) differently because of the different therapeutic ranges used in different laboratories. This result indicated the need for urgent measure to provide a uniform basis of laboratory control of clinical dosage of anticoagulant drug. 5) British Comparative Thromboplastin was found to be sueful for calibration of home-made and commercial thromboplastins. Nationa(U.K.) Reference Lab Standardized Reagent was more sentive than other partial thromboplastin reagents in detection of abnormality. 6) Except for Prothrombin time and Partial thromboplastin time, the results were evaluated by histogram, showing very wide distribution of results among participants. This indicates that these times have much room for improvement.