Abstract
A substance produced by various types of cells can induce from single hematopoietic cells the formation of colonies of normal macrophages and granulocytes. This inducer has been purified 600-fold from serum-free conditioned medium from a tissue culture line of mouse cells. It is shown that the inducer is a protein, with a molecular weight of 65-70,000, whose inducing activity was about 1 ng per colony. Acrylamide gel electrophoresis of this material gave four bands, only one of which contained inducing activity. The purified protein was inactive. Activity was regained by the addition of a low molecular weight cofactor that is present in conditioned medium. Under certain conditions, activity was also regained by the addition of adenosine 3':5'-cyclic monophosphate (cyclic AMP).