A two-dimensional structural model was devised for the Opc outer membrane protein invasin which contains 10 transmembrane strands and five surface-exposed loops. One continuous epitope recognized by three monoclonal antibodies was localized to the tip of loop 2 by synthetic peptides and site-directed mutagenesis while a second, discontinuous epitope recognized by a fourth antibody was localized to loops 4 and 5 by insertion mutagenesis. These monoclonal antibodies are bactericidal and inhibit adhesion and invasion. Most of the T-cell epitopes defined by Wiertz et al. (1996) were localized to the transmembrane strands. Oligonucleotides encoding a foreign epitope (nabla) from Semliki Forest virus were inserted into BglII restriction sites created by site-directed mutagenesis. The nabla epitopes inserted in all five predicted loops were recognized on the cell surface of live Escherichia coli bacteria by a monoclonal antibody and are exposed while nabla epitopes in the N-terminus or three predicted turns were not. The results thus confirm important predictions of the model and define five permissive sites within surface-exposed loops which can be used to insert foreign epitopes.