Anti-murine (m) interleukin-5 (IL-5) antibody was found to inhibit eosinophil (Eo) colony formation stimulated by recombinant human (rh) IL-5, but did not inhibit the production of Eo stimulated by rh IL-3 or granulocyte-macrophage colony-stimulating factor (GM-CSF). Conditioned medium (CM) prepared from eosinophilic patients' T cells with interleukin-2 (IL-2) stimulation (T-IL-2-CM), was found to contain CFU- Eo growth-stimulating factor. Using anti-mIL-5 antibody, we demonstrated that T-IL-2-CM from patients with eosinophilia contained a significant amount of IL-5. We also detected IL-5 mRNA in T cells from eosinophilic patients with IL-2 stimulation. These results suggest that IL-5 plays an important role in the induction of selective eosinophilia in humans and that IL-5 is produced from T cells with IL-2 stimulation.