The efficacy of various cholera vaccines in eliciting an intestinal antibody response was assessed in human volunteers who received oral live, oral killed, or parenteral cholera vaccines, or placebo. The intestinal immune response in terms of antibacterial and antitoxin antibodies was determined 2 and 4 weeks after immunization. By means of the mouse peritoneum opsonization assay and the infant mouse protection test, antibacterial activity could be detected in the intestinal secretions of volunteers who had been immunized either orally or by the parenteral route. Significant protective activity and duration of immunity were observed with the oral killed vaccine. The bacteriological data indicated the absence of significant intestinal colonization of the live attenuated strain after oral administration, and probably explains the observed lack of effectiveness of the oral vaccine compared with that of the killed vaccine. The predominant immunoglobulin class of intestinal antibody was found to be IgA. None of the vaccines used in the study elicited significant antitoxin activity in the intestinal secretions, as determined by the skin permeability neutralization test.