There is recent interest in the role of monocyte/macrophage subpopulations in pathology. How the hemopoietic growth factors, macrophage‐colony stimulating factor (M‐CSF or CSF‐1) and granulocyte macrophage (GM)‐CSF, regulate their in vivo development and function is unclear. A comparison is made here on the effect of CSF‐1 receptor (CSF‐1R) and GM‐CSF blockade/depletion on such subpopulations, both in the steady state and during inflammation. In the steady state, administration of neutralizing anti‐CSF‐1R monoclonal antibody (mAb) rapidly (within 3–4 days) lowered, specifically, the number of the more mature Ly6Clo peripheral blood murine monocyte population and resident peritoneal macrophages; it also reduced the accumulation of murine exudate (Ly6Clo) macrophages in two peritonitis models and alveolar macrophages in lung inflammation, consistent with a non‐redundant role for CSF‐1 (or interleukin‐34) in certain inflammatory reactions. A neutralizing mAb to GM‐CSF also reduced inflammatory macrophage numbers during antigen‐induced peritonitis and lung inflammation. In GM‐CSF gene‐deficient mice, a detailed kinetic analysis of monocyte/macrophage and neutrophil dynamics in antigen‐induced peritonitis suggested that GM‐CSF was acting, in part, systemically to maintain the inflammatory reaction. A model is proposed in which CSF‐1R signaling controls the development of the macrophage lineage at a relatively late stage under steady state conditions and during certain inflammatory reactions, whereas in inflammation, GM‐CSF can be required to maintain the response by contributing to the prolonged extravasation of immature monocytes and neutrophils. A correlation has been observed between macrophage numbers and the severity of certain inflammatory conditions, and it could be that CSF‐1 and GM‐CSF contribute to the control of these numbers in the ways proposed.