Granulocyte macrophage-colony stimulating factor (GM-CSF) was originally defined by its ability to generate in vitro granulocyte and macrophage colonies from bone marrow precursor cells. Apart from its physiological role in the control of alveolar macrophage development, it now appears more likely that its major role lies in its ability to govern the properties of the more mature myeloid cells of the granulocyte and macrophage lineages, particularly during host defence and inflammatory reactions. This review summarizes the in vivo evidence to support this proposition. This evidence includes both the findings obtained by administration of GM-CSF, e.g. as an adjuvant, and also includes those observed in depletion studies, e.g. during inflammatory reactions where GM-CSF can be shown to have a proinflammatory action.