The effect of granulocyte-macrophage colony stimulating factor (GM-CSF) (recombinant, mammalian, glycosylated, Sandoz, Schering Plough; 4 micrograms/kg every 12 h for 3 d, s.c.) on platelet activating factor (PAF, 1-O-alkyl-2-acetyl-sn glycero-3 phosphorylcholine) production from neutrophils was studied in five cancer patients with normal haemopoiesis. Peripheral blood counts, PAF production and lyso-PAF: acetyl transferase (EC 2.3.1.67) (AT) activity in neutrophils were evaluated before treatment, during treatment and 3 d after treatment had been discontinued. GM-CSF induced a three-fold increase in the number of circulating neutrophils. Neutrophils obtained during treatment produced about twice as much PAF than before treatment in response to a variety of stimuli (N-formyl-methionyl-leucyl-phenylalanine, tumour necrosis factor-alpha, phagocytosis of baker's yeast spores opsonized with C3b). This increased PAF synthesis and release is concomitant with a 2-3-fold increase in AT activity. Moreover, lower concentrations of stimuli are sufficient to induce PAF synthesis from neutrophils obtained during GM-CSF treatment. Three days after treatment had been discontinued, stimulus induced PAF production had returned to baseline levels. Since GM-CSF induces a marked shift to the left in the Arneth score, the increased PAF release might have been due to the presence of younger granulocytes. This was, however, ruled out by experiments showing that normal neutrophils primed in vitro with GM-CSF produce more PAF when challenged with the same stimuli. The potential relevance of this effect of GM-CSF treatment lies on the crucial role of PAF in inflammatory reactions and its intervention in some immune reactions, including delayed hypersensitivity, and in endotoxic shock. Lastly, increased PAF production from neutrophils may explain some toxicities observed during treatment with high doses of GM-CSF.