The feeding of protein antigens to mice results in a state of tolerance when feeding is followed by parenteral immunization. Cholera toxin (CT) is a protein that has been used extensively as a potent oral immunogen for mucosal IgA responses, but CT feeding also stimulates a substantial plasma IgG antibody response. This latter finding prompted us to study whether or not CT induces oral tolerance. Mice were fed 5 mg keyhole limpet hemocyanin (KLH) or 10 micrograms CT at least twice before parenteral immunization with 1 microgram KLH or CT in alum i.p. Plasma and intestinal secretions were collected at intervals. The specific IgG or IgA antibody in the samples was measured by ELISA. Although KLH feeding did induce oral tolerance, CT feeding did not induce oral tolerance in any of three mouse strains tested or at any dose of CT given orally. The feeding of the B subunit of CT did not result in oral tolerance either. When both CT and KLH were fed together, CT was able to abrogate oral tolerance to KLH, an antigenically unrelated protein. Moreover, feeding CT along with KLH stimulated secretory IgA anti-KLH responses, whereas no such IgA responses were found when KLH was given alone. Thus, in these experiments with protein antigens, IgA immunization and oral tolerance were reciprocally linked and did not occur simultaneously. CT appears to abrogate oral tolerance and to stimulate secretory IgA responses by altering the regulatory environment in gut-associated lymphoid tissue, shifting it toward responsiveness.