Determinants of the capacity of live Vibrio cholerae O1 isolates to evoke specific immune responses in intestinal mucosa were studied in rabbits, using mucosal immunoglobulin A (IgA) antitoxin as the measured immune response. Antitoxin responses were evoked mostly by the primary inoculation and were dose dependent; secondary-type responses were modest and occurred only when the booster inoculum was large, i.e., 10(10) CFU. The efficiency of mucosal immunization correlated closely with the mucosal colonizing capacity of the infecting strain and was otherwise independent of toxin genotype (A+ B+ or A- B+) or whether the strain was motile or nonmotile. Live bacteria evoked mucosal antitoxin more efficiently than did purified cholera toxin. Prior immunization with a nontoxinogenic (A- B-) V. cholera strain interfered significantly with the induction of mucosal antitoxin by subsequent immunization with its fully toxinogenic (A+ B+) parent. These results demonstrate the marked efficiency with which live V. cholerae stimulate a specific enteric mucosal secretory IgA response. They support the view that mucosal colonization aids efficient delivery of bacterial antigens to responsive subepithelial lymphoid tissue. This might occur by transfer of colonizing bacteria through M cells into Peyer patches or by efficient delivery of secreted toxin to M cells by mucosa-associated organisms. Preexisting antibacterial immunity interferes with colonization, which may prevent efficient antigenic stimulation and which may explain the relatively weak response to booster immunization. The same process may also limit the efficacy of hybrid enteric bacterial vaccines when there is preexisting mucosal immunity to the carrier organism due to either natural exposure or prior immunization with another vaccine that uses the same carrier.