Eosinophils contain several preformed cytokines within their specific granules. Therefore, without requiring them in de novo synthesis of cytokines, eosinophils can release quantities of granule-derived cytokines by highly regulated mechanisms. However, eosinophil "degranulation" is poorly understood, in part, because available methodologies did not appear appropriate for analyzing vesicular mobilization and transport of eosinophil granular contents. The EliCell assay is a microscopic methodology substantially modified from other techniques employed to detect cytokine release (i.e., ELISPOT). The method is a dual antibody capture/detection system in which viable eosinophils are incubated in a solid streptavidin-conjugated agarose matrix, which contains a biotinylated capture antibody against the cytokine of interest. Released cytokine is detected around non-permeabilized eosinophils with a separate fluorochrome-labeled detection antibody. Thus, the EliCell system captures and detects extracellular cytokines at the site of their release from eosinophils. As examples, we have used EliCell essays to detect the selective release of either IL-4 or IL-12 cytokines found preformed in eosinophils-from eotaxin- or anti-CD9-stimulated eosinophils, respectively. With appropriate pairs of antibodies, any preformed cytokine found into eosinophil granules could be studied and the mechanisms of their secretion evaluated by using the EliCell assay.