The anticoagulant heparin, discovered by Howell and Holt in 1916, has recently been shown to be effective in the prevention of thrombosis of veins produced by various means in experimental animals.1When the work on heparin in the University of Toronto was commenced in 1929, it appeared that two main problems had to be solved before the effect of this substance could be studied in human subjects in whom thrombosis was feared. The first was the elimination of toxic material associated with the active principle. The products which were available at that time were not sufficiently innocuous to permit of their injection in large amounts over long periods of time to human subjects. In fact, products much purer than those which were then available have since been shown to possess toxic properties. The problem of purifying the active substance was attacked by our colleagues Charles and Scott,2working