The GM/SO cell line bears a high level of stem cell factor receptors (SCF-R) i.e. c-kit protein, and therefore constitutes a potential model for studying the regulation of this crucial receptor on myeloid cells. In this study we evaluated the effect of tumor necrosis factor alpha (TNF-alpha) on the expression of SCF-R by flow cytometry. In contrast to 1 hour of preincubation, the experiments carried out after 24 hours preincubation revealed that TNF-alpha, if added alone, reduced the density of SCF-R on GM/SO cells in a dose-dependent manner. However, if combined with GM-CSF, which per se downregulates the SCF-R on these cells as well, TNF-alpha antagonized the effect of GM-CSF and slightly increased the density of SCF-R. Yet the cells incubated for 24 hours in medium without cytokines invariably expressed a higher level of SCF-R than the cells incubated in the presence of TNF-alpha and GM-CSF, either alone or in combination. In contrast to these cytokines, stem cell factor (SCF), which was also tested simultaneously in all experiments, downregulated its own natural receptor on these cells also after a preincubation of 1 hour. Furthermore, prolonged exposure of GM/SO cells to TNF-alpha for 5-7 days yielded a monocyte-macrophage-like morphology of some cells as these cells displayed an apparent glass and plastic adherence. In contrast, no such morphological changes could be observed in the presence of GM-CSF or SCF.