The correlation between heparin and mast cells was first recognized by Jorpes (7) in 1937, and the history of heparin has since generally been considered to be the history of mast cells. After this correlation was made, however, heparin-like material with lower anticoagulant activity was found by several investigators as a by-product of heparin isolation from various tissues, such as lung and liver (6, 16). This material, called heparitin sulfate, contained less sulfate per disaccharide unit than heparin and contained a mixture of N-sulfated and N-acetylated glucosamine units (13). Both heparin and heparitin sulfate have been shown to be covalently linked to protein and can therefore by called proteoglycans.