Monoclonal antibodies have proved to be highly specific tools for defining the antigenic epitopes of Pseudomonas aeruginosa outer membrane macromolecules. In this article we have highlighted the use of monoclonal antibodies in the study of lipopolysaccharide heterogeneity and in particular have demonstrated that single monoclonal antibodies can recognize epitopes on lipid A which are conserved in all Gram negative bacteria tested. Monoclonal antibodies against P. aeruginosa outer membrane proteins have been used to demonstrate the strong conservation of specific antigenic sites in all P. aeruginosa strains tested. In the case of one monoclonal antibody, specific for outer membrane lipoprotein H2, the antigenic site recognized by the antibody was also found to be conserved in all group 1 Pseudomonads. The implications of these monoclonal antibodies to bacterial taxonomy is discussed. Monoclonal antibodies against two separate conserved surface epitopes on outer membrane protein F were isolated and differentiated according to their reactions with 2 mercaptoethanol-reduced protein F and with proteolytic and cyanogen bromide peptide fragments of protein F. One of these protein F-specific monoclonal antibodies has been demonstrated to have immunotherapeutic potential.