1) Colonization and diarrhea in the rabbit due to RDEC-1 strain E. coli elicits an immune response that prevents subsequent infection with the same bacterium. 2) RDEC-1 and InvEC strain colonization of the gut of pregnant rabbits is followed by the appearance, post-delivery, of anti-InvEC or RDEC-1 strain s-IgA in the milk, some of which was able to agglutinate as many as 3.4 X 10(11) bacteria per mg of s-IgA. 3) Immune s-IgA, given by the orogastric route to rabbits, is able to survive the digestive enzymes of the gastrointestinal tract to prevent the colonization and diarrhea that follows the inoculation of InvEC and RDEC-1 strains. One possible explanation for the protective effect of s-IgA is the in vivo agglutination of bacteria in the lumen of the gastrointestinal tract.