The process of cleaving one protein with another is a very basic one in biology. In many organisms mature proteins are released from their precursor forms by the action of proteases. Proteolysis is used extensively in other important processes such as the coagulation cascades and in the biosynthesis of biologically active peptides. Peptide biosynthesis is characterized by the limited, specific digestion of substrates at a variety of types of cleavage sites. Analysis of the genes coding for glandular kallikreins, a group of presumptive prohormone processing enzymes, has enabled elucidation of the complete structure of the enzymes and determination of sites of gene expression. The physical properties of these enzymes and their interaction with their substrates provide a unique opportunity to analyse structure-function relationships.