The basis of differentiation of Pseudomonas aeruginosa into the 17 serotypes of the International Antigenic Typing Scheme is differences in an outer membrane glycolipid, lipopolysaccharide (LPS). This observation, together with the high toxicity and pyrogenicity of LPS, has led to the search for alternative “common” antigens for use as vaccines. The relation between the major outer membrane proteins of serotype strains was studied in three ways. By demonstrating conservation of outer membrane protein receptors for bacteriophages, a high similarity of outer membrane protein patterns on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels, and antigenic cross-reactivity of major outer membrane proteins, it was shown that the major outer membrane proteins were closely related. Radioiodinated antibodies to outer membrane proteins interacted with outer membrane proteins after SDS-PAGE separation and electrophoretic blotting of the separated outer membrane proteins onto nitrocellulose paper. This demonstrated that major outer membrane proteins F, H2, and I were antigenically related in all serotype strains,