The anti-infectious role of human milk may be, at least partly, ascribed to its content in secretory IgA. As lectins are present in various infectious antigens, the binding of different types of IgA to three lectins (concanavalin A, peanut agglutinin, wheat germ agglutinin) was studied by Elisa. The specificity of those bindings was assessed by inhibitory experiments performed with the corresponding oligosaccharides. The following were found for the three lectins: (1) the lectin-binding capacity of colostrum secretory IgA was markedly greater than that of normal plasma IgA1 (p less than 0.001); (2) the lectin-binding capacity of polymeric IgA1 was greater than that of monomeric IgA1 (p less than 0.001). This property of mucosal IgA may be responsible of a nonimmune opsonization able to prevent the early step of some infectious mucosal diseases, i.e. the attachment of bacteria to epithelial cells by lectin-like bonds and also the penetration into the body of some antigens able to favor the development of allergy. Milk mucosal IgA, present in significant amounts in human colostrum and mature milk - but not in infant formulas - may therefore play an important polyvalent protective role in newborns.