These studies were designed to determine whether colony stimulating factor (CSF) and diffusible granulocytopoietic stimulator (DGS) represented a single substance which stimulates the production of granulocytes in vitro and in vivo. The effect of endogenous CSF upon DGS activity was studied after injection of 40 µg endotoxin. This resuited in both DGS activity and high serum CSF levels, but the time of appearance and duration of the two activities followed different patterns. In particular, CSF peaked and was declining 24 hr following endotoxin at which time DGS was not yet detectable. Then, as serum CSF fell, the DGS activity rose. These data support the concept that the DGS activity detectable with short term Millipore chamber cultures is not due to CSF and suggests the effects of CSF in vitro and DGS in vivo are due to different factors.