The effect of amphotericin B (AmB) treatment on the mononuclear phagocyte system of mice was investigated. Peritoneal macrophages from mice that received AmB treatment showed a higher phagocytic and antibacterial activity than those from normal untreated mice. When the levels of macrophage precursor cells in bone marrow and spleen were followed in mice after AmB treatment, an eightfold increase in the splenic content of limited stem cells for both macrophages and granulocytes (colony forming units in culture) and a threefold increase in the number of pluripotent hemopoietic stem cells (colony forming units in spleen) were observed on day 4. These were also accompanied by a slight increase in the colony forming units in spleen and in culture in femoral marrows. AmB was capable of inducing a large number of peritoneal colony forming cells in the peritoneum, and caused a significant rise in the serum levels of colony stimulating factor. No significant change in the level of blood monocytes was noted, although a transient increase in the proportion of neutrophils was observed within 24 h after AmB treatment.