Culture of WEHI-3B myelomonocytic leukemic cells in semi-solid agar medium containing serum from mice injected with endotoxin serum (ES) led to the development of maturing granulocytes and macrophages in most leukemic colonies. ES contains high levels of granulocyte-macrophage colony-stimulating factor (GM-CSF), a regulator known to stimulate differentiation of these leukemic cells, but an antiserum which neutralized greater than 85% of the GM-CSF in ES did not suppress the differentiation-inducing activity of ES on WEHI-3B cells. The active factor in endotoxin serum stimulating differentiation in WEHI-3B leukemic cells (GM-DF) was separated from most of the GM-CSF by gel filtration using Ultrogel AcA44. The residual CSF associated with the GM-DF appeared to stimulate selectively granulocytic colonies. Disproportionation of GM-DF and GM-CSF was observed in ES fractions obtained using concanavalin-A/Sepharose chromatography: none of the GM-DF bound to this matrix, whereas 40% of the GM-CSF bound and was eluted with competing alpha methylglucopyranoside. Although no separation of GM-CSF and GM-DF was obtained using DEAE-Sepharose, non-isoelectric focusing in amphoteric buffers indicated charge differences between the differentiation factor and several sub-species of GM-CSF. Sequential purification of GM-DF from ES using 40 - 70% ammonium sulfate precipitation gel filtration and phenyl-Sepharose chromatography resulted in a 25-fold purification. In all fractionation procedures used, a sub-species of GM-CSF, stimulating granulocyte colony formation, was consistently associated with partially purified GM-DF, but some subspecies of GM-CSF clearly lacked any capacity to induce differentiation in the leukemic cells. The observations suggest that the factor in post-endotoxin serum most efficient in enforcing differentiation in myelomonocytic leukemic cells may be a subset of GM-CSF molecules with a selective capacity to stimulate granulocyte colony formation by normal cells.