We previously identified Mannheimia haemolytica outer membrane proteins (OMPs) that may be important immunogens by using immunoproteomic analyses. Genes for serotype 1-specific antigen (SSA-1), OmpA, OmpP2, and OmpD15 were cloned and expressed, and recombinant proteins were purified. Objective 1 of this study was to demonstrate immunogenicity of the four recombinant OMPs in mice and cattle. Objective 2 was to determine if the addition of individual recombinant OMPs or combinations of them would modify immune responsiveness of mice to the recombinant chimeric protein SAC89, containing the main epitope from M. haemolytica outer membrane lipoprotein PlpE and the neutralizing epitope of M. haemolytica leukotoxin. Mice vaccinated with recombinant OmpA (rOmpA), rSSA-1, rOmpD15, and rOmpP2 developed significant antibody responses to M. haemolytica outer membranes and to the homologous recombinant OMP. Cattle vaccinated with rOmpA and rSSA-1 developed significant antibodies to M. haemolytica outer membranes by day 28, whereas cattle vaccinated with rOmpD15 and rOmpP2 developed only minimal responses. Sera from cattle vaccinated with each of the recombinant proteins stimulated complement-mediated killing of the bacterium. Concurrent vaccination with SAC89 plus any of the four rOMPs singly resulted in increased endpoint anti-SAC89 titers, and for the SAC89/rSSA-1 vaccinees, the response was increased significantly. In contrast, the SAC89/P2/SSA-1 and SAC89/OmpA/P2/D15/SSA-1 combination vaccines resulted in significant decreases in anti-SAC89 antibodies compared to SAC89 vaccination alone. In conclusion, under the conditions of these experiments, vaccination of mice and cattle with rOmpA and rSSA-1 stimulated high antibody responses and may have protective vaccine potential.The major cause of severe bacterial pneumonia in cattle is Mannheimia haemolytica serotype 1 (S1), and current vaccines against M. haemolytica are only moderately efficacious (9, 36). Shewen and Wilkie (43) demonstrated that immunity against M. haemolytica requires antibodies against leukotoxin (LKT), which causes necrosis, apoptosis, or activation of ruminant leukocytes, as well as antibodies against bacterial cell surface antigens. The important surface immunogens needed to stimulate protective immunity against M. haemolytica appear to be outer membrane proteins (OMPs) (13,33,34,37,38).Our laboratory demonstrated that high antibody responses to a major 45-kDa outer membrane lipoprotein, PlpE, correlated with resistance against experimental challenge, and PlpE proteins were nearly identical among serotype 1 and serotype 6 isolates (2). Cattle vaccinated with commercial M. haemolytica vaccines to which 100 g of recombinant M. haemolytica S1 PlpE (rPlpE) was added had significantly greater resistance against experimental challenge with either S1 or S6 than did cattle vaccinated with the commercial vaccine alone (11, 12). The major epitope region of M. haemolytica S1 PlpE, designated region 2 (R2), consists of 8 imperfect hexapeptide repeats of QAQNAP loc...