Secretory antibodies against bacteria and viruses in human colostrum and milk are known to be important protective factors for the breast-fed infant. The authors have shown by enzyme immunoassay that colostrum contains IgA and IgM antibodies to a number of autoantigens: native DNA, actin, myosin, myoglobin, laminin, transferrin and thyroglobulin. These antibodies were polyspecific-those with anti-DNA reactivity immunopurified on a DNA-cellulose affinity column bound to a panel of self- and environmental antigens. The levels of natural autoantibodies in the immunoglobulin fraction of human colostrum were 3-10 times lower (when presented as antibody activity per microgram of immunoglobulin) than in the immunoglobulin fraction of serum. The biological significance of the presence of B cells with autoantibody specificity in the mammary gland and of natural autoantibodies in colostrum and milk is not clear. It has been suggested that self-reacting autoantibodies in serum play a major role in the selection of the pre-immune B-cell repertoire and in the maintenance of the immune homeostasis. The authors hypothesize that the natural autoantibodies in colostrum and milk may contribute to the selection process of physiological repertoire during the early postnatal period in breast-fed infants. This could explain the lower frequency of allergic, inflammatory and autoimmune diseases and lymphomas which is seen in their later life when compared with that observed in children who have been formula-fed after birth.