Rats born of egg albumin immunized mothers have a diminished capacity to produce IgE antibody to egg albumin persisting for at least 13-14 weeks after birth. At the same time the primary IgG response to the antigen is usually enhanced. Previous studies indicated that these effects were mediated by factors transferred in maternal milk. The phenomenon can be duplicated by the administration of small quantities of immune serum to rats during the first 3 weeks of life. The active component of immune serum is shown to be specific antibody. Suckling rats acquire egg-albumin-specific IgG from the immune mother via the milk. Their serum level approaches that of the mother by 20 days but declines rapidly after weaning to become undetectable by 6-8 weeks. As maternal influence on the immune responsiveness of the offspring persists for several weeks beyond this time, it is unlikely that the mechanism involves a simple blocking by circulating antibody of the access of antigen to cellular receptors. Alternative mechanisms are briefly discussed. Attention is drawn to the possibility that the suppression of IgE antibody responsiveness by maternal IgG may represent a physiological regulatory process with a role in inhibiting the development of infantile allergies.