The presence of a CSF enhancing activity in the serum of endotoxin-treated mice.

Serum taken from mice a few hours after injection of endotoxin is a potent source of a stimulator of in vitro myelopoiesis. By means of dose-response studies, the biological activity of this material was compared to that of a colony stimulating factor (CSF) from pregnant mouse uteri. Postendotoxin serum appears to contain two different activities: a stimulating activity which may be identical to CSF and an activity which augments the action of CSF. The separate nature of the two activities is demonstrated by differences in the rate at which they are diluted out and by differences in the time at which they are maximally present after indotoxin administration. It is therefore concluded that the colony-stimulating properties of postendotoxin serum are not due solely to CSF present in the serum.