EoL-1 cells differentiated into mature eosinophil-like cells when incubated with butyric acid (BA). The differentiated cells possessed many granules that stained with Luxol fast blue and acquired the ability to produce leukotriene C4 and eosinophil cationic protein (ECP), and migrated toward some eosinophillotactic attractants, including PAF, IL-5, IL-3 and GM-CSF. Monoclonal antibody to GM-CSF and IL-5, especially GM-CSF, strongly decreased the ability of BA to induce differentiation, whereas antibody to IL-3 did not. Recombinant human (rh) IL-3, rhGM-CSF and recombinant mouse (rm) IL-5, by themselves, did not have the ability to induce differentiation. On the other hand, the presence of these cytokines significantly (p < 0.01) augmented BA-induced cell differentiation. Other cytokines including rhIL-1 alpha/beta, rhIL-2, rhIL-4, rhIL-6, rhIL-8, rhTNF-alpha and rhIFN-gamma had no effect on differentiation. Expression of GM-CSF and GM-CSF receptor mRNA was detected in cellular RNA obtained from BA-stimulated EoL-1 cells. Moreover, about 5-10 pg/ml of GM-CSF was detected in the culture supernatant of BA-stimulated EoL-1 cells. These findings suggested that BA-induced EoL-1 cell differentiation was elicited under the influence of GM-CSF endogenously produced in these cells. GM-CSF also seemed to be an autocrine differentiating factor in EoL-1 cells.