Previous work showed that lactoferrin was involved in the bacteriostatic effect of human milk on E. coli O111. Further experiments on the general nature of this effect have been carried out together with an examination of the role of IgA. Milk samples from different individuals differed in their ability to produce bacteriostasis of three pathogenic serotypes of E. coli. The bacteriostatic effect was stable to heating at 60 degrees for 35 min. As in the case of serum, the bacterial iron transporting compound, enterochelin, abolished the bacteriostatic effect of human milk. IgA was isolated from the milk samples in two forms which appeared to differ in molecular weight. When mixed with lactoferrin, some of these fractions induced bacteriostasis which could be reversed by Fe3+. Since the fractions were devoid of bactericidal activity in the presence of complement it appeared that IgA was involved in the induction of bacteriostasis. It was also concluded that the mechanism of bacteriostasis was identical in serum and milk. These results are discussed in relation to both the protective effect of feeding colostrum and milk and also the resistance of the adult gastrointestinal tract to infection.