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We used competitive panning to select a panel of 10 different human antibodies from a large semisynthetic phage display library that distinguish between serum complement-resistant and complement-sensitive strains of the gram-negative diplococcus Moraxella (Branhamella) catarrhalis. Western blotting analyses and inhibition enzyme-linked immunosorbent assays showed that all phage antibodies were directed against the same or closely spaced epitopes on the target protein, which is the high-molecular-weight outer membrane protein (HMW-OMP) of M. catarrhalis. HMW-OMP was found in multiple isolates of complement-resistant but not complement-sensitive M. catarrhalis strains. Nucleotide sequence analysis demonstrated that the immunoglobulin heavy-and light-chain variable-region genes encoding the 10 phage antibodies were remarkably similar, with a strong preference for basic amino acid residues in the heavy-chain CDR3 regions. This is the first report showing that competitive panning is a successful procedure to obtain phage antibodies against differentially expressed structures on phenotypically dissimilar strains of prokaryotic cells.