The in vitro growth of hemopoietic colonies, in the presence of an active colony stimulating factor (CSF), provides a useful model for evaluation of potential humoral factors controlling granulopoiesis. The present investigations were undertaken to assess the relationship between rapidly induced granulocytopenia in rats and the presence of heightened CSF activity. Sera from control animals showed virtually no activity (0-3 colonies). Significant levels of CSF (40-60 colonies) were demonstrated within 2-6 hr after rabbit antirat neutrophil serum (ANS). The increased activity corresponded with the phase of rapid neutrophil destruction. However, in animals rendered neutropenic by pretreatment with cyclophosphamide, ANS administration was also associated with a prompt rise in CSF levels; the magnitude of this response approximated that seen in animals treated with ANS alone. These results suggest that heightened CSF may accompany destruction of relatively small numbers of neutrophils. The temporal appearance of heightened CSF activity in relation to induced granulocytopenia is consistent with the concept that this factor may be a regulator of granulopoiesis.