Maternal antibodies (IgG and IgA) not only provide passive protection against microbial infections, but also exert a variety of equally important active, idiotypically-mediated immunoregulatory functions. Since the generation of maternal antibodies depends entirely on the stimulation of the mother's immune system by external mainly thymus-dependent antigens, with long-lived antigen independent plasma cells in the bone marrow, maternal antibodies represent the mother's collective ontogenetic immunological experience. Although their stimulatory potential in mice is restricted to the neonatal imprinting period, maternal antibodies exert a life-long determinative influence which is even dominant over seemingly genetic predispositions. Therefore, the functional impact of maternal IgG antibodies appears phenotypically as a non-genetic inheritance.