Abstract: A method for assaying the proteinase inhibitor content of tissue of a plant. The protease inhibitor and other protein products are extracted from the plant tissue by preparing a mixture of solvent and comminuted plant tissue to form a solid fraction and a liquid fraction which contains the protease inhibitor and other protein products. The liquid fraction is heated to a temperature and for a time period sufficient to denature at least some of the other protein products without substantially denaturing the protease inhibitor. The denatured, undesired protein products are removed by centrifugation or filtering to prepare a clarified extract solution. The amount of protease inhibitor present in the clarified extract is measured. If the assay is used in the selection of plant tissues for use in the extraction of proteinase inhibitor, plant tissues with a proteinase inhibitor content less than a pre-selected standard are rejected.