AbstractThe natural inhibitors of proteolytic enzymes are proteins. These inhibitors associate reversibly with the enzymes to form stoichiometric protein‐protein complexes, in which substrate‐analogous association at the active center of the enzyme results in competitive inhibition of all catalytic functions. The very widespread occurrence of inhibitors in the animal and plant kingdoms underlines their biological importance in the intermediate metabolism, which can be understood as an extension of the possibilities for temporary and local limitations of enzyme activities. Existing knowledge includes a series of covalent structures, detailed kinetic data on the reversible protein‐protein interaction, the processes involved in inactivation, and chemical methods for the modification of these proteins. Early X‐ray structural data for an inhibitor and its enzyme complex provide an insight into the molecular structure of the latter and the interactions involved in the association to form the