These proceedings contain the papers presented at the Bayer-Symposium V which was held in Grosse Ledder on October 16-21, 1973 and which was simultaneously the Second International Research Conference on Proteinase Inhibitors. The cordial atmosphere, the frank discussions and the profitable exchange of ideas during the First International Research Conference on Proteinase Inhibitors which was held on November 4–6, 1970 in Munich were a stimulant for us to consider holding a Second Conference especially as the steadily increasing research activities in this and related fields called for a new assessment of recent efforts in proteinase inhibitor research. In the three years since the First Conference was held, this field has developed particularly in the areas of molecular aspects of protein-protein interactions and in the extension to inhibitors of nonserine proteinases and peptidases. In planning this meeting the Organizing Committee was faced with the problem of selecting a limited number of participants and we decided that it was appropriate and necessary to focus in detail on the chemistry of proteinase inhibitors, their interac- tions with a variety of enzymes and also to invite more general papers to survey the physiological and clinical aspects of the subject which we felt would be of importance in the future.