To unambiguously demonstrate the immunotherapeutic potential of outer membrane porin protein F from Pseudomonas aeruginosa, a series of monoclonal antibodies have been isolated and demonstrated to be specific for protein F by Western blotting procedures. The antibodies recognize a surface-exposed antigenic site that is conserved on all Pseudomonas aeruginosa strains tested to date. One of these monoclonal antibodies named MA4-4 resulted in passive protection against subsequent infections by Pseudomonas aeruginosa in two different mouse infection models. In vitro studies using human polymorphonuclear leukocytes suggested that this antibody opsonized Pseudomonas aeruginosa for phagocytosis. The data suggest that immunotherapy based on porin protein F has definite potential for success.