Salmonella typhimurium and Escherichia coli contain at least three broad specificity aminopeptidases: peptidases A, B, and N. Both peptidases A and N have been sequenced and shown to belong to different structural families (A to the leucineaminopeptidase (LAP) family and N to alanyl aminopeptidase family). With the aim of understanding the unique physiological roles of these peptidases, we have cloned and sequenced the pepB genes of S. typhimurium and E. coli. These genes code for a 427 amino acid protein (for the S. typhimurium enzyme predicted molecular mass:46,362, observed (mass spectroscopy): 46,360). The two sequences are 91% identical to each other and define peptidase B as a member of the LAP family (31.9% amino acid identity to bovine lens LAP and 34.54% amino acid identity to E. coli peptidase A). In common with most other members of this family, PepB is a metalloenzyme composed of six identical subunits. Using overproducing strains, PepB has been purified to homogeneity. Studies underway are aimed at comparing the specificities and other properties of PepA and PepB.