The opsonic components of breast milk responsible for phagocytosis of surface-adherent Staphylococcus aureus by human polymorphonuclear leukocytes were investigated. There was significantly greater phagocytosis of bacteria pre-opsonized with 100% breast milk than of unopsonized bacteria (p less than 0.001). Heat inactivation of breast milk had no effect on surface phagocytosis, indicating that phagocytosis is independent of complement. The predominant immunoglobulin in breast milk, secretory immunoglobulin A (IgA), did not promote phagocytosis. In contrast, IgG, which is present in very low amounts in breast milk (0.05 mg/ml), was as opsonic as 100% breast milk, suggesting that this is the major opsonin. An oxidative burst as measured by chemiluminescence was observed during phagocytosis of bacteria pre-opsonized with 100% breast milk. Heat inactivation of breast milk reduced the chemiluminescence response to the level of control. Neither secretory IgA nor IgG stimulated a polymorphonuclear leukocyte chemiluminescence response to surface-adherent bacteria. These experiments indicate that IgG is the principal component of breast milk responsible for surface phagocytosis but that complement is required for the generation of chemiluminescence and thus may be essential for intracellular killing of bacteria. Secretory IgA, despite its abundance in breast milk, has no effect on surface phagocytosis or neutrophil chemiluminescence.