Human neutrophils were activated by the bacterial chemotactic peptide N-formylmethionyl-leucyl-phenylalanine (fMLP) to produce superoxide (O2-) and to release the primary granule enzyme beta-glucuronidase and the predominantly secondary granule enzyme lysozyme. Pretreatment with granulocyte-macrophage colony-stimulating factor (GM-CSF) increased the secretion of all three substances upon addition of fMLP. The augmentation by GM-CSF was significantly attenuated by the 5-lipoxygenase inhibitor AA861 and by the guanylate cyclase inhibitor LY83583. The secretion induced by fMLP alone was much less affected by either of the two inhibitors. AA861 inhibited leukotriene B4 production in neutrophils primed with GM-CSF and stimulated with fMLP, and LY83583 inhibited GM-CSF-evoked increases of 3',5'-guanosine monophosphate. The data suggest that activation of lipoxygenase and guanylate cyclase is not critical to the fMLP stimulation pathway, but they may be important components of the pathway by which GM-CSF augments neutrophil responses to fMLP. However, AA861 and LY83583 may have important actions in addition to inhibition of 5-lipoxygenase and guanylate cyclase.