Granulocyte colony-stimulating factor (G-CSF) is a strongly conserved glycoprotein which induces the survival, proliferation and differentiation of neutrophilic granulocyte precursor cells and functionally activates mature blood neutrophils. Among the family of colony-stimulating factors, G-CSF is the most potent inducer of terminal differentiation to granulocytes and macrophages of leukemic myeloid cell lines. The actions of G-CSF are mediated through a small number of specific cellular receptors which are induced to internalize and accumulate within the cell after binding of G-CSF at 37 degrees C. G-CSF receptors can be lost from the cell surface not only by binding G-CSF, but indirectly by other factors including GM-CSF, Multi-CSF, bacterial lipopolysaccharides, chemotactic peptides and phorbol esters, depending on cell type. Possible relationships of these processes to the mechanism of action of G-CSF are discussed.