Eosinophils are a numerically dominant cell population within the schistosome granuloma. These granuloma eosinophils can produce a variety of cytokines, including IL-2, IL-4, IL-5, and IFN-gamma. Therefore, eosinophils may play a key role in the determination of the unique cytokine microenvironment within the granuloma milieu. These studies investigated the potential role of eosinophils in the regulation of granuloma immunopathology. We have characterized spleen- and granuloma-derived eosinophils based on cellular activation and cytokine production during the development of murine schistosomiasis. Based on the criteria of hypodensity and CD69 expression, granuloma eosinophils were highly activated and very homogeneous at 7 and 11 wk postinfection. Splenic eosinophils were also activated at 7 wk postinfection, but were much more heterogeneous than their granuloma counterparts. By 11 wk postinfection, few hypodense splenic eosinophils were observed. Eosinophils represented the majority of cytokine-producing cells in the granuloma and were a dominant source of IL-4. Eosinophils also produced IL-2, IL-5, and IFN-gamma, using the criteria of mRNA in situ hybridization and intracellular cytokine staining by FACS. Granuloma eosinophil activation and cytokine production were greatest at the time of maximum granuloma formation, i.e., 10-12 wk after initial cercarial exposure. Therefore, locally activated eosinophils, not Th2 lymphocytes, produce the majority of Th2 cytokines in the granuloma milieu and may be important determinators of immunopathology in schistosomiasis.