Soybean trypsin inhibitor (SBTI) is a reversible competitive inhibitor of trypsin and other trypsin-like proteases such as chymotrypsin, plasmin and plasma kallikrein. This immobilized presentation of SBTI binds free trypsin tightly with high pH (4.5-8.0) and allows for a simple gradient to low pH (~2.5-3.0) for elution.