Abstract: The present invention describes methods for screening or selecting novel proteolytic enzymes. These enzymatic functions can be selected from isolated chromosomal DNA libraries or from pools of mutagenized DNA encoding a proteolytic function. In particular, the selection method of the present invention, a phage gene is chosen that encodes a gene product necessary for the production of a phage. The phage carrying the modified gene is introduced into a host. Also, DNA libraries present within a cloning vector are introduced into host cells. The host cells are grown under conditions where the introduced libraries are expressed in the host cells. The presence of an enzymatic function capable of cleaving the target peptide is identified on the basis of production of infective phage.