Acinetobacter baumannii has emerged as a highly troublesome, global pathogen. Treatment is complicated by high levels of antibiotic resistance, necessitating alternative means to prevent or treat A. baumannii infections. We evaluated an immunotherapeutic approach against A. baumannii, focusing on the surface polysaccharide poly-N-acetyl-beta-(1-6)-glucosamine (PNAG). We used a synthetic oligosaccharide of 9 monosaccharide units (9Glc-NH2) conjugated to tetanus toxoid (TT) to induce antibodies in rabbits. In the presence of complement and polymorphonuclear cells, antisera to 9Glc-NH2-TT mediated the killing of A. baumannii St, a high-PNAG-producing strain, but not its isogenic PNAG-negative, in-frame deletion mutant strain, S1 Delta pga. Complementing the pgaABCD locus in trans in the shuttle vector pBAD18kan-ori, plasmid Delta pga-c, restored the high levels of killing mediated by antibody to PNAG observed with the wild-type SI strain. No killing was observed when normal rabbit serum (NRS) or heat-inactivated complement was used. Antiserum to 9Glc-NH2-TT was highly opsonic against an additional four unrelated multidrug-resistant clinical isolates of A. baumannii that synthesize various levels of surface PNAG. Using two clinically relevant models of A. baumannii infection in mice, pneumonia and bacteremia, antisera to 9Glc-NH2-TT significantly reduced levels of A. baumannii in the lungs or blood 2 and 24 h postinfection, respectively, compared to levels of control groups receiving NRS. This was true for all four A. baumannii strains tested. Overall, these results highlight the potential of PNAG as a vaccine component for active immunization or as a target for passive antibody immunotherapy.