AbstractWEHI‐3 cell‐conditioned medium with the capacity to stimulate megakaryocyte colony formation was separated by Sephadex G‐150 column chromatography. The development of colonies containing megakaryocytes was observed only when mixing experiments were performed. Individual fractions did not support megakaryocyte colony growth. The two factors in WEHI‐3 CM required for megakaryocyte colony growth had apparent average molecular weights of 35,000 daltons (megakaryocyte CSF) and 100,000 daltons (megakaryocyte potentiator). The results were confirmed in serum‐free conditions in which colonies were directly identified in the cultures by acetylcholinesterase staining. Two growth factors may be necessary for the genesis of megakaryocytic
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