The intestinal immune response to cholera toxoid was studied in dogs and rats. Oral or intraperitoneal priming followed by duodenal boosting with toxoid reulted in antitoxin-containing plasma cells (ACC) in jejunal lamina propria of rats. Priming and boosting by the intraperitoneal route alone induced almost no jejunal response. Lamina propria ACC were derived largely from migrating immunoblasts, which appeared earlier among thoracic duct lymphocytes. Protection of dogs after repeated subcutaneous immunization with toxoid was mediated largely, or entirely, by serum-derived antibody. The sequence of subcutaneous priming and repeated oral boosting induced longer lasting protection mediated almost entirely by the local intestinal immune mechanism. The results suggest that cholera immunization should be specifically designed to stimulate the intestinal immune mechanism and that the subcutaneous oral immunizing sequence may be an efficient way to do this.