Oral immunization was shown to reduce the passage of bacterial antigen (BA) through the intestinal wall. The extent of the reduction found was dependent on the technique used for antigen assay, but with 125I-BA in an everted gut sac system, the amount of transported antigen was less than 60% of the amount in a control non-immune system. The inhibitory effect was due to coproantibodies which probably function by complexing and trapping the antigen in the mucous layer. This inhibition of antigen transport seems a prerogative of IgA antibodies since serum-derived antibodies caused concomitant adverse effects.