Colony-stimulating activity.

Hemopoietic cells, when placed in single-cell suspension in a semisolid growth medium for seven to 14 days, have been found to form visible colonies of granulocytes and monocytes. Colony growth appears only in cultures to which a source of colony-stimulating activity (CSA) is added. Because of these in vitro effects, CSA is thought to be an in vivo regulator of granulocyte and monocyte production from progenitor cells in the bone marrow.