The biological actions of IgA antibodies in bile are largely undefined. We therefore tested whether biliary IgA antibodies could specifically inhibit cholera enterotoxin (CT)-induced secretion in the rat ileum. Rats were immunized by CT given orally or by injection into Peyer's patches. Bile was collected by bile duct cannulation, and anti-CT antibodies in the bile were measured by ELISA. CT plus bile from either immunized or unimmunized rats, or CT plus anti-CT-containing bile which had been absorbed by a CT immunosorbent, were instilled into in situ ileal loops in unimmunized rats; CT alone, or buffer was instilled into other loops. The bile used from the immunized rats contained IgA, but neither IgG nor IgM, anti-CT antibodies. It was found that bile containing IgA anti-CT antibodies almost totally inhibited the secretory effect of CT, and this inhibition was abrogated by absorption of the IgA anti-CT antibodies. Thus, IgA antibodies to an enterotoxin, secreted into bile, are effective against the enterotoxin in the rat intestine in vivo.