AbstractThe murine myelomonocytic leukemia WEHI‐3B exists as differentiation‐inducible (D+) and noninducible (D−) cell lines. Both lines produce a CSF species that stimulates exclusively the formation of neutrophil granulocyte colonies. This G‐CSF copurifies with a mast cell growth factor but can be separated from M‐ and GM‐CSF. NZB bone marrow is unresponsive to G‐CSF stimulation. WEHI‐3B D+cells can be induced to terminal granulocyte differentiation by a factor present in murine and human postendotoxin serum that is different from G‐CSF present in WEHI‐3B D+or D−CM since the latter has little or no leukemia differentiation‐inducing activity. Endotoxin treatment ofC. parvumprimed mice leads to simultaneous induction of serum activities with selective action on myeloid leukemic cells, a serum differentiation inducing activity and a leukemic colony inhibitory activity. These factors act synergistically to block leukemic stem cell self‐renewal. The results suggest that a variety of inducible factors may have potent and select