Interleukin 4 (Il-4) is an immunoregulatory cytokine which induces T-cell proliferation and differentiation into a Th₂ phenotype, and is of particular importance for the induction of IgE synthesis. In the present study, the capability of human peripheral blood eosinophils from allergic and non-allergic donors to produce Il-4 was examined. Using reverse transcribed polymerase chain reaction (RT-PCR), it was shown that highly purified eosinophils from allergic patients express mRNA for Il-4. Resting eosinophils also gave specific immunoreactivity with anti-Il-4 antibodies, consistent with translation of Il-4 mRNA. Light and electron microscopic immunocytochemistry revealed that Il-4 was prestored in the eosinophilic granules. These results were confirmed by Il-4 specific ELISA which showed that Il-4 production could be upregulated in the eosinophils and released from the eosinophils following stimulation with the calcium ionophore A23187. These data indicate that eosinophils may be an important source of Il-4 at sites of allergic inflammation. Thus, eosinophils may act as immunomodulatory cells enhancing the allergic response through formation of Th₂-cells and inducing the isotype switching to IgE in human B-cells.