Previous studies showed that after 5 μg of Salmonella typhosa endotoxin there was an increase in colony stimulating factor temporally related to a fall in murine marrow in vitro colony forming cells (CFC). This was followed by differentiation along the marrow granulocytic pathway. The present studies showed that after 5 μg of endotoxin the peripheral blood CFC fell by approximately 50% at one hour, rose to a level ten fold that of control at six hours and then returned to control values by 48 hours. There was a progressive increase in the number of splenic CFC to ten fold that of control from 24 to 72 hours after endotoxin. These data imply a migration of CFC from the marrow to the spleen along with an in‐situ increase in splenic CFC. Thus, either migration or differentiation may explain the fall in marrow CFC after endotoxin. Spleen colony forming units (CFU) in the marrow were measured by a transplantation technique and the transplantation fraction (f Fx) determined. A decrease in marrow CFU at 24 hours after endotoxin was secondary to a change in the f Fx. from 11.1% to 7.6%. There was however, an increased percentage of CFU in DNA synthesis in the interval of 6–48 hours after endotoxin, as judged by the hydroxyurea technique. As the marrow CFC fell within 20 minutes of endotoxin administration, the data suggest the CFC may be affected initially and that changes in the generative cycle of the CFU may be of a secondary nature.