Human milk may contribute to protection against gram-negative infection by promoting intraluminal agglutination or killing of bacteria or by preventing bacterial attachment to epithelial surfaces. This paper explores the effect of human milk on the sensitivity of faecal E. coli to serum bactericidal activity and agglutination specificity, factors which have been regarded as related to virulence. Faecal E. coli isolated from breast-fed infants differed from those from formula-fed infants in two respects: They were more sensitive to the bactericidal effect of human serum and more often spontaneously agglutinating. E. coli strains isolated from sources outside the gastro-intestinal tract, that is the prepuce and female peri-urethral region, were in breast-fed babies less sensitive than faecal strains. The findings are compatible with the hypothesis that a breast-milk factor favours the proliferation of mutant strains. The observed effects of breast-milk might be associated with decreased bacterial virulence, and be one of the ways in which breast-feeding protects against infection.