CrataBL, a protein isolated from Crataeva tapia bark, which is both a serine protease inhibitor and a lectin, has been previously shown to exhibit a number of interesting biological properties, including anti-inflammatory, analgesic, anti-tumor, and insecticidal activities. Using a glycan array we have now shown that only sulfated carbohydrates are effectively bound by CrataBL. Since this protein was recently shown to delay clot formation by impairing the intrinsic pathway of coagulation cascade, we considered that its natural ligand might be heparin. Heparin is a glycosaminoglycan (GAG) that interacts with a number of proteins including thrombin and antithrombin III, which have a critical, essential pharmacological role in regulating blood coagulation. We have thus employed surface plasmon resonance to better understand the binding interaction between the heparin polysaccharide and CrataBL. Kinetic analysis shows that CrataBL displays strong heparin-binding affinity (KD= 49 nM). Competition studies using different size heparin-derived oligosaccharides showed that the binding of CrataBL to heparin is chain length dependent. Full chain heparin with 40 saccharides, or large oligosaccharides, having 16 to 18 saccharide residues, show strong binding affinity for CrataBL. Heparin-derived disaccharides through tetradecasaccharides show considerably lower binding affinity. Other highly sulfated GAGs including chondroitin sulfate E and dermatan 4,6-disulfate showed comparable CrataBL binding affinity to heparin. Less highly sulfated GAGs, heparan sulfate, chondroitin sulfate A and C and dermatan sulfate displayed modest binding affinity as did chondroitin sulfate D. Studies using chemically modified heparin show that N-sulfo and 6-O-sulfo groups on heparin are essential for CrataBL- heparin interaction.