Six monoclonal antibodies directed against the trimeric form of outer-membrane pore protein PhoE of Escherichia coli K-12 were isolated and characterized. All six antibodies bind to PhoE protein in intact cells and as isolated trimers, but not to dodecyl-sulphate-denatured monomers. Cross-reaction with the related pore proteins OmpF protein and OmpC protein was not observed. A hybrid pore protein in which the amino-terminal 74 amino acids of PhoE protein have been replaced by the corresponding part of OmpF protein is able to bind the six monoclonal antibodies. Five of the antibodies bind to the PhoE proteins of thirteen different Enterobacteriaceae when expressed in E. coli K-12, whereas the other antibody recognizes PhoE protein from nine of these strains. Four monoclonal antibodies are able to block adsorption of the PhoE-protein-specific phage TC45 to its receptor on whole cells. None of the antibodies has any effect on the uptake rate of the antibiotic cefsulodin through PhoE protein pores. Five antibodies are able to direct the complement-mediated killing of PhoE-protein-carrying cells. It is concluded that the six monoclonal antibodies recognize at least three distinct cell-surface-exposed epitopes whose specificity is determined by the carboxy-terminal 256 amino acids of PhoE protein.