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We studied whether gut mucosal IgA antitoxin production as well as the acquired protection against cholera toxin (CT) after oral immunization with CT are both thymus-dependent immune manifestations. In contrast to normal BALB/c mice, nude, athymic mice did not respond to oral immunizations with CT with either IgA antitoxin-producing cells (SFC) in the lamina propria or protection against challenge with CT in ligated intestinal loops. However, when nude mice were first reconstituted by grafting of syngeneic thymus glands, both IgA antitoxin SFC in the lamina propria and protection were stimulated by oral immunizations with CT and the responses were of similar magnitude to those of normal mice after immunizations. During in vitro culture, isolated lamina propria lymphocytes from immunized but not from control mice concomitantly and proportionally produced IgA antitoxin and CT-neutralizing activity. We conclude that intestinal antitoxin formation and protection against toxin challenge after oral immunization with CT are both critically thymus-dependent and therefore likely to be under T-cell control.