Pasteurella multocida OmpA (PmOmpA) belongs to the major and multifunctional Escherichia coli OmpA family of proteins. We have previously reported that the protein is conserved, immunogenic and an adhesin that binds host cells and host cell extracellular matrix molecules [Dabo SM, Confer AW, Quijano-Blas RA. Molecular and immunological characterization of Pasteurella multocida serotype A:3 OmpA: evidence of its role in P. multocida interaction with extracellular matrix molecules. Microb Pathog 2003;35(4):147-157]. In this study, we found that immunization of mice with the recombinant PmOmpA elicited strong Th2-type immune response, characterized by high immunoglobulin G(1) (IgG(1)) antibodies production. Subsequent intraperitoneal homologous challenge of the immunized mice resulted in lack of protection associated with the high IgG(1) titers in anti-rPmOmpA sera. Furthermore, the protection afforded by vaccination with P. multocida OMPs alone was adversely affected by the addition of the rPmOmpA to the vaccine preparations. The results demonstrate that PmOmpA has a detrimental effect on the efficacy of vaccination with OMPs in mice. Targeted inactivation of pmOmpA gene in P. multocida 232 represents a potential mean towards the development of an effective vaccine candidate.