Liposomes prepared from phosphatidylinositolmannosides (extracted from BCG) and cholesterol are efficiently endocytosed by macrophages. Phagocytosis of particles or microbes modifies macrophage metabolism and in some cases, delivers potent stimulating signals to macrophages. We examined the effect of phosphatidylinositolmannoside-based liposomes on three macrophage functions especially important for host defenses: nitric oxide production, oxidative burst and TNF-alpha secretion. Phosphatidylinositolmannoside-based liposomes, added as empty vesicles, induced a strong NO synthase activity in mouse peritoneal macrophages primed either by interferon-gamma or by trehalose dimycolate. They also induced a moderate production of TNF-alpha. Phosphatidylinositolmannosides conferred activating properties to pH-sensitive liposomes. In contrast, liposomes composed of phosphatidylcholine and phosphatidylserine were unable to activate primed macrophages.