An ideal vaccine does not yet exist to prevent cholera, a significant health problem in many less developed countries. Vibrio cholerae, the agent of epidemic and endemic cholera, colonizes the small bowel and secretes a potent enterotoxin that consists of a single A subunit, which stimulates adenylate cyclase activity, and five identical B subunits which bind to the ganglioside GM1 receptor of intestinal mucosal cells. Previous studies in man indicate that toxoid-derived antitoxic immunity by itself is insufficient to provide effective, long-lasting protection against cholera. Using recombinant DNA techniques we have now constructed a live, attenuated V. cholerae strain by deleting genes encoding the enterotoxin. Restriction enzyme fragments encoding cholera toxin were deleted in vitro from cloned vibrio chromosomal DNA and the resulting mutations introduced into the chromosome of a vibrio strain of proven immunogenicity. Recently, Mekalanos and coworkers have reported attenuated V. cholerae strains constructed by similar methods. It appears that recombinant DNA techniques offer a promising approach to the development of effective cholera vaccines.