Breast milk macrophages culturedin vitrosynthesized and secreted increasing amounts of protein, lysozyme, and prostaglandin E2(PGE2) into the extracellular medium. These cells were also shown to actively phagocytose labeled zymosan particles in culture. Morphologic characteristics, phagocytosis, and secretory responses of the macrophages were altered depending on the presence of various stimuli in the culture. Concanavalin A, endotoxin and zymosan particles, but not latex particles, all resulted in an increased PGE2secretion into the medium. Although total protein synthesis was not altered by any of these stimuli, Concanavalin A and endotoxin resulted in a decreased lysozyme concentration in the extracellular medium. Concanavalin A enhanced, whereas endotoxin and prior phagocytosis of latex particles inhibited phagocytosis of labeled zymosan particles. These findings indicate that phagocytosis and secretions of milk macrophages may be altered depending on the nature of the stimulating agent.