Locomotion and oxidative metabolism of polymorphonuclear leukocytes from 15 patients receiving recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) were examined in vitro. At the end of each GM-CSF treatment course, polymorphonuclear leukocyte (PMN) chemotactic responses were suppressed and no enhancement of formyl-peptide-stimulated superoxide production was observed. The priming of PMN superoxide production normally seen after in vitro GM-CSF exposure was also blunted in these cells. By using control donor PMN, two membrane-fluidizing agents, pentoxifylline and butanol, were shown to normalize suppressed PMN chemotaxis caused by in vitro GM-CSF (1 nM) exposure. Pentoxifylline, but not butanol, also reversed the effects of in vitro GM-CSF on PMN superoxide production. When PMN obtained from six patients at the end of GM-CSF therapy were exposed to pentoxifylline in vitro, the chemotactic suppression typically observed was significantly improved. The data suggest that GM-CSF may affect PMN function via mechanisms involving membrane fluidity or cell deformability or both.