Dialyzed urine of mice bearing leukocytosis-inducing fibrosarcoma stimulated granulocyte colony formation in semisolid agar culture of human bone marrow cells. Removal of phagocytic cells prior to stimulation did not interfere with the formation of these colonies in the culture. On the other hand, macrophage colonies were predominantly produced when murine bone marrow cells were stimulated by the dialyzed mouse urine. The activity of colony-stimulating factor (CSF) in the urine of normal mice was less than 1/100 of that in the urine of tumor-bearing mice. DEAE-cellulose column chromatography separated the activity stimulating human granulocyte colony formation from that stimulating murine macrophage colony formation. Further purification showed that a sialoglycoprotein with an apparent molecular weight of 80,000 corresponded to the macrophage CSF, which was devoid of activity toward human cells. The molecular properties of the human-active granulocyte CSF could not be studied further, because it was quite unstable.