Abstract
Cells of the differentiation‐responsive mouse myelomonocytic leukemia cell line WEHI‐3B D^+^ form colonies in agar exhibiting a low frequency of spontaneous differentiation mainly in the macrophage pathway. Compared with undifferentiated colonies, spontaneously differentiating colonies have a reduced content of clonogenic cells and surviving clonogenic cells tend themselves to form differentiating colonies, both being characteristics of differentiated colonies induced by the regulator, granulocyte colony‐stimulating factor, G‐CSF. Colony crowding increased the frequency of spontaneously differentiating colonies and WEHI‐3B D^+^ colony cells were shown to release material able to induce differentiation in WEHI‐3B D^+^ colonies. Cells from spontaneously differentiating D^+^ colonies were not hyperresponsive to the induction of differentiation by G‐CSF and did not release larger amounts of differentiation‐inducing material than did cells from undifferentiated colonies. Cells of the differentiation‐unresponsive WEHI‐3B D^−^ line produced similar amounts of differentiation‐inducing material to those produced by D^+^ cells. Apparently spontaneous differentiation in WEHI‐3B D^+^ colonies seems most likely to be due to exposure of the colony‐forming cell or its ancestors to a differentiation‐inducing factor of WEHI‐3B origin prior to culture in agar, the genetic program initiating differentiation being inherited by the progeny of the exposed cell.