The avirulent, A-B+, streptomycin-resistant mutant designated Texas Star-SR, isolated from a virulent, hypertoxinogenic, colonizing strain of Vibrio cholerae (Ogawa serotype, El Tor biotype) and administered intragastrically or intraduodenally in adult rabbits, has been found to induce substantial immunity to subsequent challenge (in ligated intestinal loops) with virulent wild-type cholera vibrios (of both homologous and heterologous biotype and serotype). Significant resistance to challenge with one strain of human heat-labile enterotoxin (LT)-producing Escherichia coli was also demonstrated, but resistance against two other human LT-producing strains was either nil or marginal under these experimental conditions. Significant, but not striking, resistance against challenge with purified choleragen was obtained, whereas protection against a bolus challenge of purified porcine LT was not statistically significant.