Partial hepatectomy (hepx) of the adult animal induces a substantial hepatic erythropoietin (Ep) response following hypoxia and results in a transient reinitiation of hepatic erythropoiesis. This phenomenon is associated with an elevation in the number and activity of the hepatic macrophages or Kupffer cells. Kupffer cells have been implicated in the production not only of Ep, but of the moiety which regulates granulocyte growth in vitro, Colony Stimulating Factor (CSF). Histological examination of the regenerating liver revealed some foci of granulopoiesis, but not to the extent noted for erythropoiesis. Granulopoiesis was not associated with islet conformations involving central reticular or macrophagic cells. In addition, serum derived from the hepatic venous drainage of hepx animals possessed moderate levels of CSF, as determined by its ability to promote the growth of granulocytic-macrophagic colonies in agar culture.