Piglets suckled by dams that had been vaccinated with K88 antigen were significantly more resistant to deaths caused by neonatal diarrhea after challenge with a large dose of a K88-positive enteropathogenic strain of Escherichia coli than piglets suckled by control dams. The factors most likely to be involved in protection of the piglets were investigated by comparing the antibacterial activities of serum and mammary secretions from the two groups of dams. Vaccination stimulated the production of K88 antibodies, which were associated with anti-adhesive activity directed against the adhesive properties of the K88 antigen, and of O8 antibodies; the latter antibodies were attributed to traces of O8 antigen in the vaccine. Neutralizing activity against heat-labile enterotoxin was present in several dams before vaccination but was not stimulated by bacteriostatic activities were similar in serum and mammary secretions from both groups of dams and appeared to play no significant role in the protective after parturition were atrributed to exposure of the dams to the challenge strain excreted by the piglets. It was concluded that neutralization of the adhesive properties of K88 antigen by K88 antibodies in colostrum and in milk contributed significantly to the protection of piglets from vaccinated dams. However, the contribution of antibacterial activities associated with the greater levels of O8 antibodies in colstrum from the vaccinated group cannot be entirely excluded.