Enterokinase is a serine proteinase in the duodenum that plays a critical role in mammalian digestion. It converts trypsinogen into its active form trypsin, by cleaving its aminoterminal hexapeptide Val(Asp)4-Lys. More recently, the enterokinase has been shown to have a broad utility in cleaving fusion proteins produced in Escherichia coli. The enzyme is particularly suitable for this role because of its high degree of specificity, its tolerance to a wide range of reaction conditions, and the fact that its recognition sequence lies entirely on the aminoterminal side of the scissile bond. This enzymatic activity allows release of carboxyl-terminal fusion partners from fusion proteins without leaving unwanted amino acid residues on their amino termini.