Progeny mice were confronted with maternal gamma-globulin of a different allotype by either back-cross mating, intercross mating, or by foster nursing. In all cases, many mice subsequently produced alloantibodies directed against the incompatible maternal type of IgG(2a)-globulin. In one series of experiments, immunologic tolerance to the maternally derived gamma-globulin was demonstrated to exist in the period before formation of spontaneous antibody. The state of tolerance was then lost, unless maintenance injections of foreign gamma-globulin were given. These studies demonstrate in a natural situation that maternally derived foreign proteins can first induce a state of immunological tolerance which is followed, after disappearance of the antigen, by a state of immunity. As such, this parallels the experimental induction of tolerance to foreign proteins by neonatal injections.