Eosinophils are not only the source of cytotoxic and proinflammatory mediators but they can also generate cytokines and growth factors, including their own factors of differentiation, namely IL-3, GM-CSF, and IL-5. Synthesis of IL-5 by eosinophils was demonstrated by in situ hybridization and immunostaining in a variety of diseases, such as coeliac disease, asthma, hypereosinophilic syndrome, or skin diseases. However, IL-5 synthesis by eosinophils was not shown in Crohn's disease, whereas in other diseases, it was restricted to a subpopulation of eosinophils, suggesting some heterogeneity in cytokine-producing eosinophils. Here, we report that human eosinophils, in addition to the synthesis of IL-5, and Th2 cytokine, can synthesize IFN gamma, a Th1 cytokine, as well as IL-10 and IL-4, known to be mainly produced by Th2 cells. Double immunostaining procedures reveal the coexpression of IL-5, IL-4, and IL-10 by the same eosinophil populations, different from IFN gamma-producing eosinophils. We propose that distinct subpopulations of human eosinophils express Th2 or Th1 cytokines. These results point to the importance of cytokines derived from non T cells in the regulation of the immune response.