This paper elaborates and extends earlier reports from this laboratory on the mechanisms by which coproantibodies protect against enteric bacterial infection. Purified antibody preparations obtained from the lumen of the intestine of orally vaccinated germfree mice, containing very small amounts of S-IgA but no other serologically active immunoglobulins, were found to protect mice against experimental enteric cholera infection. This protection was associated with a reduction in that fraction of the total vibrio population which was adsorbed to the mucosa. Total counts of intestinal vibrio populations were not affected. The mechanism of this process was investigated by using metabolic inhibitors in experimental infections of intestinal loops of rabbits. Iodoacetate reversed the antibody-mediated shift of the vibrio population away from association with the mucosa. This suggests that the mucosal cell may furnish some accessory factor which, in conjunction with antibody, results in an antibacterial mechanism on the mucosal surface. Iodoacetate had no effect on total vibrio population and adsorption of vibrios in nonimmunized intestinal loops. Consequently, a direct effect of iodoacetate on the vibrios, rather than on the mucosal cells, is a less likely explanation for the observed effects.