Bone marrow colonies: stimulation in vitro by supernatant from incubated human blood cells

A substance which stimulates growth of granulocytic and mononuclear cell colonies from mouse and human bone marrow was produced by incubated human blood cells. It is resistant to heat and freezing and is not dialyzable. Intact irradiated and unirradiated cells had very little activity, and sonically disrupted cells had no activity. The addition of plasma or sonically disrupted cells to the cell supernatant decreased its activity.