Purified populations of both human peripheral blood monocytes and murine peritoneal macrophages synthesize and release Prostaglandin E in vitro. In contrast, prostaglandin E was detected in neither the supernate fluids from cultures of highly enriched human lymphocytes and granulocytes, nor in nonadherent murine peritoneal cells. Macrophage prostaglandin E production was markedly enhanced by endotoxin, and completely suppressed by indomethacin. All neoplastic monocyte-macrophage cell lines examined elaborated prostaglandin E in vitro, either constitutively or after induction with endotoxin. In contrast, prostaglandin E production could not be detected from either a T- or B-cell lymphoma, whether or not they were treated with endotoxin. These findings thus indicate that the blood monocyte and tissue macrophage represent an important source of prostaglandin E, a function shared by both normal and neoplastic mononuclear phagocytes.