The O-sialoglycoprotein endopeptidase (glycoprotease, Gcp) is secreted by Pasteurella haemolytica A1, a Gram-negative pathogen associated with bovine pneumonic pasteurellosis. When the cloned gcp gene is expressed in Escherichia coli, the recombinant glycoprotease (rGcp) is exported to the periplasm but does not exhibit enzymatic activity. Polyclonal calf sera and murine monoclonal antibodies to rGcp were used for the further immunological and biochemical characterization of the authentic and recombinant Gcp. The results showed that the gcp gene product is the sole component of Gcp activity. Homologues to the P. haemolytica A1 Gcp were detected by Western immunoblot analysis in a number of Gram-negative bacteria, including E. coli. However, the secretion of Gcp with O-sialoglycoprotein endopeptidase activity appears to be restricted to P. haemolytica A serotypes.