Since Jorpes1 by his work at Karolinska Institutet in Stockholm in 1935 defined the chemical nature of heparin and made this physiologic anticoagulant of Howell available in a pure form, he and his co-workers, as well as other Swedish investigators, have been trying to determine the clinical serviceability of this preparation. Their efforts have been successful. Their results and those of the group in Toronto, Canada, working on the same problem have been summarized in two monographs, by Jorpes2 and Mason.3 One of the most important questions was whether heparin could be used in treating thromboembolic disease. Some valuable work on this problem has been done independently by Crafoord4 and Crafoord and Jorpes5 in Stockholm and by Murray and Best in Toronto. In several studies, based on 500 cases, Crafoord has shown that in no instance has postoperative thrombosis appeared in patients who have been