To study the capacity and regulation of cytokine production by normal peripheral blood eosinophils, we isolated eosinophils from healthy individuals and stimulated them with immobilized Ig or TNF-alpha, with or without exogenous IL-5. By reverse transcription-PCR, uncultured, freshly isolated eosinophils constitutively expressed mRNA for IL-4, IL-10, and TGF-beta1. Eosinophils stimulated by immobilized secretory IgA, immobilized IgA, immobilized IgG, or TNF-alpha for 3 h expressed mRNA encoding IL-3, IL-4, IL-8, IL-10, granulocyte-macrophage CSF, TNF-alpha, TGF-beta, and RANTES. The mRNA for IL-2, IL-5, or IFN-gamma was not detected. IL-4 and IL-10 protein, but not IL-8, were measurable in lysates of fresh eosinophils or eosinophils cultured with medium alone for 24 h. Eosinophils incubated with immobilized Ig or TNF-alpha released IL-8 protein into the supernatants. In contrast, IL-4 and IL-10 proteins were not detectable. Soluble secretory IgA immune complexes also induced degranulation, as measured by eosinophil-derived neurotoxin, and IL-8 release, but not IL-4 or IL-10 release, from eosinophils. Release of IL-8 protein and storage of IL-4 and IL-10 proteins were enhanced by exogenous IL-5 and inhibited by a transcription inhibitor, actinomycin D. Degranulation of stored granule proteins was not affected by actinomycin D. Therefore, normal peripheral blood eosinophils can transcribe and synthesize several cytokines, including IL-4, IL-8, and IL-10; some are stored, and some are released. These cytokines may play important roles in modulating immune responses in diseases associated with eosinophils.