The regulation of granulocyte function by cell-derived factors is emphasised in this review. The most important of these cell-derived factors belongs to a class of substances known as colony stimulating factors (CSF) so named because they stimulate the maturation of progenitor cells into clumps or "colonies" of mature cells. CSF mediated regulation of granulocytes is likely to be important for the two reasons (i) the site of CSF production can determine the site of granulocyte activation and (ii) the type of CSF produced can determine the type of granulocyte activated. Blood mononuclear cells were found to be a good source of granulocyte activating material suggesting that interaction between these two cell types is important in vivo. The potential clinical use of CSF-like substances is discussed.