Two major outer membrane proteins of Pasteurella multocida, designated OmpH and OmpA, were characterized and shown to be related to the families of porin and heat-modifiable proteins, respectively. The backpack hybridoma tumor system in BALB/c mice was used to continuously deliver immunoglobulin G2b (IgG2b) monoclonal antibodies (MAbs) specific for OmpH (MAb MT1) and OmpA (MAb MT4.1). MAbs were detected in serum and peritoneal lavage samples of mice bearing hybridoma tumors by an enzyme-linked immunosorbent assay and an immunoblot assay. Highly significant protection was observed in mice bearing MT1 hybridoma tumors against both intraperitoneal and intranasal challenge infections with homologous nontoxigenic P. multocida strains possessing MAb MT1-reacting epitopes, whereas the mice bearing MT4.1 hybridoma tumors were not protected. The numbers of P. multocida organisms in the lungs of mice bearing MT1 hybridoma tumors were significantly less than those in lungs of mice bearing MT4.1 hybridoma tumors at 48 h postchallenge. These results indicate that the OmpH-specific MAb inhibited proliferation of P. multocida in the lungs. MAb MT1 was unable to kill P. multocida in vitro in the presence of complement. However, an enhanced phagocytosis by polymorphonuclear cells (PMNs) was observed in mice bearing MT1 hybridoma tumors. P. multocida induced a more extensive and rapid influx of PMNs into the peritoneal cavity of mice bearing MT1 hybridoma tumors than of mice bearing MT4.1 hybridoma tumors. The results of this study demonstrate for the first time that IgG MAbs against OmpH of P. multocida are involved in the protection of mice against lethal challenge infection by means of opsonization and inhibition of proliferation of P. multocida as a result of increased influx of PMNs into the infection site.