SummaryA technically simple, rapid method for the quantitative titration of trypsin or similar proteolytic enzymes, and of protease inhibitors, suitable for critical tests on a large scale is described. The procedure utilizes technics familiar to serologists, rather than to skilled biochemists only, and gives definitive, reproducible results that can be expressed in simple figures. A feature is the provision for the making of a permanent record of each titration. Since the reagents are stable, and readily standardized, comparable findings can be obtained in separate experiments conducted in the same or in different laboratories at any time.The assistance of Mrs. David A. Cook with records and statistics is gratefully acknowledged.