Mouse bone marrow cells were cultured in vitro under conditions that allowed little granulopoiesis but permitted proliferation of mononuclear phagocytes. During 10 days’ culture, the population gradually shifted from a preponderance of undifferentiated blast cells and promonocytes to macrophages. Cells up to the A (immature) macrophage stage were capable of DNA synthesis and mitosis. B (mature) macrophages were nondividing. Blast cells were not phagocytic and lacked glass adhesiveness and demonstrable surface receptors for IgG immunoglobulin. With progressive cellular maturation, peroxidase activity disappeared after the monocyte stage. Glass adhesiveness, phagocytic ability, and surface receptors for immunoglobulins appeared to increase with cell maturation from the promonocyte through the macrophage stage of development.