Mice ingesting milk from immune mothers are protected against infection with Giardia muris. In order to be protected, the infant mice must be suckling immune milk at the time of challenge with the parasite and must continue to do so for at least three to five days thereafter. Suckling immune milk does not induce sustained immunity; infants become susceptible to infection after weaning begins at 14 days of age. Immune milk contains specific IgA and IgG antibody to Giardia, which may be the mediators of the demonstrated protection. The properties of this protection are similar to those described for protection against other enteric infections by milk.