The objective of this study was to examine the effect of breast-feeding by immunized dams on Helicobacter colonization in newborns. Urease-based immunization regimens failed to protect nursing pups against H. felis, whereas H. felis lysate-cholera toxin resulted in protection. This observation correlated with a high recognition of cell surface-expressed bacterial antigens by milk antibodies. Protection lasted until weaning, indicating that infection is maintained at undetectable levels by passive immunity but then resumes when breast-feeding stops.