SummaryStrains ofEscherichia coliwere opsonized in human colostrum via heat stable opsonins and the classic complement pathway, but colostrum lacked capacity to opsonizeE. colivia the alternative pathway. There was no bacteriostatic activity against serum sensitiveE. colistrains, although specific antibodies against the strains were present. Neutrophils suspended in colostrum had normal chemotaxis and this was not altered by treating the colostrum with HCI.SpeculationPossibly the migration of neutrophils to the intestinal wall and lumen in response to cytotaxic factors may be enhanced by colostrum. Milk derived opsonins may enhance phagocytosis. Human breast milk may, thus, play a role in the defense against bacteria in the gut of the nursed infant and locally in the mammary gland by promoting neutrophil chemotaxis and phagocytosis.