Colicins A and E1 are two pore-forming colicins sharing homology in their C-terminal domains but not in their N-terminal or central domains. Using site-directed mutagenesis, restriction sites were inserted at the proper locations to allow recombination of these domains. Six different constructs were obtained. All these proteins were expressed in Escherichia coli and properly recognized by monoclonal antibodies directed against epitopes located in different domains of colicin A. Out of the six hybrids, only two were released to the extracellular medium. Immunocytolocalization indicated that some of the hybrids aggregated within the cytoplasm. With some hybrids, the defect in release was related to a defect in synthesis of the lysis protein that normally promotes release.