The induction of a jejunal antitoxin response was studied in dogs immunized with cholera toxin or toxoid. Single doses of toxoid given subcutaneously (s.c.) or of toxin given intraluminally (i.l.) were each effective in priming the mucosal immune system, whereas toxoid given i.l. was much less effective. In contrast, toxin and toxoid given i.l. were each effective as booster antigens. The local secondary response was rapid and brief, the peak occurring at about 7 days after i.l. boosting and declining by 90% after 2 more weeks. After s.c. priming and i.l. boosting with toxoid, antitoxin-containing plasma cells appeared predominantly in the portion of jejunum exposed to the i.l. booster. The appearance of antitoxin-containing plasma cells in jejunal lamina propria correlated with the amount of antitoxin recovered in jejunal washings which, in turn, correlated with protection against challenge with cholera toxin. Thus, lamina propria antitoxin-containing plasma cells appeared to be the source of protective antitoxin. However, after sequential s.c.-oral immunization with toxoid, protection against challenge with Vibrio cholerae far outlasted the major systemic and local antitoxin responses and was not obviously explained by either. These studies reveal methods for induction of a mucosal antitoxin response, but leave in question the mechanism of prolonged protection induced by s.c.-oral immunization of dogs.