Human blood monocytes in a feeder layer or by use for conditioning medium produced a colony-stimulating factor capable of stimulating the in vitro growth of colonies of granulocytes and mononuclear cells from human and murine marrow. Lymphocytes and neutrophils did not stimulate colony formation, and medium conditioned by neutrophils was inhibitory. This suggests that the monocyte may control granulocyte proliferation and maturation.