The 5' terminus of the gene that codes for the heat-stable enterotoxin of Escherichia coli (ST) was genetically fused to the 3' terminus of the gene that codes for the binding subunit of the heat-labile enterotoxin of E. coli (LT-B). The ST-encoding gene used for these studies was constructed synthetically with appropriate restriction sites to permit in-frame, downstream insertion of the oligomer. For this construction, maximum expression of ST antigenicity was obtained when a seven-amino-acid, proline-containing linker was included between the LT-B and ST moieties. The LT-B-ST fusion peptide was purified by affinity chromatography and consisted of a single polypeptide chain with an apparent molecular weight of 18,000 when examined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. There was no evidence of multimer formation and no change in the mobility of the fusion peptide when it was boiled in SDS or in SDS with dithiothreitol. The LT-B-ST fusion peptide was nontoxic, and immunologic determinants of both LT and ST were recognized by antibodies to the native toxins. More importantly, the LT-B-ST fusion peptide was immunogenic. Animals immunized with crude or purified preparations containing the hybrid molecule produced antibodies that were able to recognize native toxin in vitro. Significantly, these antibodies were able to neutralize the biological activity of native ST.