Medium conditioned for 24 hours by mononuclear human blood cells contains an inducer of granulopoiesis lacking colony stimulating activity.

Regulation of granulocyte and macrophage formation was studied by a modified CFU-C assay. Mouse bone marrow cells were cultured in methylcellulose in vitro. After colony counting on d 7, the cells were washed out to determine the total cell number per plate, and the distribution of granulocytes and macrophages in smears. By this procedure it was possible to study pathway-specific regulators. The colony stimulating factor in medium conditioned by mouse L-cells appeared specific for the macrophage cell line; 99% of the colony cells were macrophages. Medium conditioned for 24 h by mononuclear cells from human blood, had no colony forming capacity, but increased colony size and generated significant granulocyte production when combined with L-CSF. This granulopoiesis inducing factor was thermo-labile, and was mostly retained by an Amicon filter separating molecules at 100 000 daltons.