Cell-free extracts from human polymorphonuclear neutrophils (PMN) inhibited rebound granulopoiesis in the bone marrow and spleen of mice pretreated with cyclophosphamide to remove endogenous PMN. Absolute numbers of granulocytemonocyte progenitor cells (CFU-C) and net endogenous colony-stimulating activity (CSA) production were found to be increased 3 days after cyclophosphamide in the bone marrow and 6 days after in the spleen. Administration of PMN extract to the drug-treated mice prior to rebound granulopoiesis substantially decreased CSA production and CFU-C but not spleen B lymphocyte colony-forming cells. In addition, mice treated with PMN extract had decreased levels of CSA in serum and in conditioned medium of marrow-free bone, heart, and lung cultures. Inhibition was reversed by injection of bacterial lipopolysaccharide. Extracts from PMN of patients with chronic myelogenous leukemia, inactive in vitro, had no effect in vivo. These results demonstrate that inhibitory activity derived from PMN can control granulopoiesis in vivo.