A direct comparison of GM-CSF and G-CSF in a panel of in vitro neutrophil-function assays was performed to investigate any differences in activity profiles. In our modified chemotactic assay, GM-CSF rapidly increased the migratory capacity of polymorphonuclear cells (PMNs) to move toward fMLP and LTB4. In contrast, G-CSF only stimulated PMN migration towards fMLP. GM-CSF, but not G-CSF, increased PMN cytotoxic killing of C. albicans blastospores. The expression of PMN surface antigens associated with Fc- and complement-mediated cell-binding (Fc gamma R1, CR-1 and CR-3), and adhesion signalling (ICAM-1), was increased after the exposure of GM-CSF, but not to G-CSF. In contrast these CSFs demonstrated relative equipotency in their ability to induce PMN anti-bacterial phagocytosis, and to restore the Staphylococcus aureus killing capacity of dexamethasone-suppressed neutrophils. The phagocytic activity of PMNs for opsonized yeast, as well as hexose-monophosphate shunt activity, was equivalent following GM-CSF or G-CSF treatment. We discuss the significance of the difference in activity profiles in this article.