In this work we provide evidence that granulocytes produce macrophage colony-stimulating factor (M-CSF) in the band cell stage and secrete it upon sodium caseinate-mediated differentiation to polymorphonuclear cells. We identified M-CSF in an enriched population of myeloid band cells from murine bone marrow using a chromophore-labeled monoclonal anti-M-CSF antibody. An ELISA assay was then used to detect secreted M-CSF in culture supernatants of enriched band cells differentiated to mature neutrophils using sodium caseinate. Colony formation in vitro by the supernatants from differentiating band cells was blocked by anti-M-CSF, thus suggesting that this factor is the only one responsible for this activity. Our data imply that casein can modulate hematopoiesis possibly via M-CSF production. Finally we discuss the possibility whether this M-CSF in concert with G-CSF could establish a cellular communication network between macrophages and granulocytes allowing them to simultaneously arrive at the inflammatory site. 2009 Elsevier GmbH. All rights reserved.