Previous to the introduction of heparin it had been discovered that certain mammalian tissues contain one or more anticoagulants. McLean,1 in Howell's laboratory in 1916, first isolated an active fraction which in 1918 Howell and Holt2 named heparin and which was at first isolated exclusively from the liver tissue of the dog. Later, Charles and Scott3 showed that this substance is present in varying amounts in the lung, liver and skeletal muscles of the ox, the lung containing the largest amount. During the past five years the anticoagulant properties of heparin have been available for use outside the laboratory, owing to the work of Best4 and his co-workers in Toronto, Canada, and also to that of Jorpes and his school5 in Stockholm, Sweden. Jorpes described heparin as a polysaccharide of high molecular weight which also contains nitrogen and sulfur. The purest preparation contains 40 per