Pyrrolomycin B enhanced both humoral immune response and delayed-hypersensitivity against sheep red blood cells in mice. In spleen cell culture it was a weak inhibitor of mitogenesis. However, in combination with concanavalin A, there was stimulation of mitogenesis in spleen cell culture. Pyrrolomycin B also enhanced phagocytosis of yeasts by peritoneal macrophages after in vivo administration to mice. Thus, pyrrolomycin B, formerly isolated as an antibiotic agent, is an immunopotentiator possibly acting on the membrane of lymphocytes or macrophages.