Human granulocyte-macrophage colony-stimulating factor (Gm-CSF) is an in vitro and in vivo stimulator of human bone marrow myelomonocytic precursor cells and mature granulocyte and macrophage effector cells. We have compared the effect of GM-CSF on the synthesis of 5-lipoxygenase products induced by the chemotactic peptide fMet-Leu-Phe and the calcium ionophore A23187 in human neutrophils. Although Gm-CSF alone did not stimulate detectable synthesis of products of the 5-lipoxygenase pathway, pre-incubation of neutrophils with 200 pM GM-CSF for 1 hour at 23°C enhanced synthesis of leukotriene B4, its all-trans isomers and Ω-oxidation products, and 5-hydroxyeicosatetraenoic acid in response to both the calcium ionophore A23187 (1.5 μm), and the chemotactic peptide fMet-Leu-Phe (0.1 μM). This priming effect of Gm-CSF was maximal after a 60 min incubation at 23°C, or after a 30 min pre-incubation at 37°C. The effect of GM-CSF was maximal using a concentration of 1 nM. Enhancement of the leukotriene synthesis stimulated by A23187 was only observed when the cells were stimulated by the ionophore for periods of 3 minutes or less. In contrast, the enhancing effect of GM-CSF was still apparent when cells were exposed to fMet-Leu-Phe for as long as 15 minutes. Furthermore, the enhancing effect of GM-CSF was ablated when neutrophils were stimulated with A23187 and exogenous arachidonic acid. However, co-addition and exogenous arachidonic acid with fMet-Leu-Phe did not entirely mask the effect of GM-CSF. Possible mechanisms of action of GM-CSF are discussed.