Various opinions have been presented on the merits and demerits that breastfeeding gives for the allergic onset of the babies. In this report, we focused on whether food proteins eaten by mother mice and secreted into breast milk as IgA-immune complexes contribute to the allergy prevention through oral tolerance in infants who ingest the milk. BALB/c mice were divided into two groups; E-group fed only egg white proteins and M-group fed only cow’s milk proteins as a dietary protein source. After immunizing M-group infants fed their own mother’s milk with ovalbumin/alum, diarrhea associated with experimental Th2 intestinal inflammation was induced by oral administration of ovalbumin. The diarrhea was dramatically suppressed in E-group infants. Concomitantly, low level of serum anti-ovalbuminand ovomucoid-IgG1 and IgE, suppression of IL-4 synthesis by spleen cells, and low incidence of anaphylactic death after intravenous injection of ovalbumin were observed preferentially in E-infants. Immune complexes of respective dietary proteins and IgA were found in the breast milk obtained from each group of mother. Oral administration of pseudo immune complex chemically synthesized with ovalbumin and monoclonal mouse IgA in advance effectively suppressed anti-ovalbumin-IgG1 synthesis in adult mice after immunization with ovalbumin. The tolerance induced by the pseudo immune complex of ovalbumin diminished spontaneously while mice did not take egg white proteins. Thus, immune tolerance and then prevention of allergic disorder against dietary proteins were acquired via breastfeeding by mothers feeding the relevant proteins, probably through the immune complexes of dietary proteins and sIgAs secreted into breast milk.