PIP: Field observations are reported which support an assumption that human breast milk acts benefically through induced host resistance to infection. Though the mechanisms of this induced host resistance to infection are not clearly understood, this article suggests the following factors as important: specific antibodies to infectious agents, influences stimulating or inhibiting certain intestinal microorganisms, and nonspecific antimicrobial factors. Immunoglobulins are present in human milk, with IgA representing the most abundant; the role of complement and immunoglobulin in induction of resistance to infection is well-known. Since all immunoglobulins have antibody activity, it is conceivable that immunoglobulins provide the material for antibodies to various etiological agents; for example, serum IgA contains all types of antibodies; serum IgG has a wide variety of antibodies to viruses, rickettsiae, protozoa, H antigens of Salmonella, and bacterial antitoxins and incomplete Rh antibodies; and the IgM fraction contains antibodies to O antigens of Enterobacteriacae, Rh agglutinins, and syphilis reagins. Another aspect of human milk as an inducer of host resistance is the bifidus factor which promotes development of characteristic microflora; since the bifidobacteria metabolize a variety of sugars, producing large amounts of acetic and lactic acids and trace amounts of formic and succinic acids, these organisms affect pH and, hence, certain resistance factors. Other factors present in milk include lysozyme (muramidase), complement, interferon, and immune cells, all of which promote host resistance to infection.