Sera from 10 patients with eosinophilia contained eosinophil colony stimulating factor (Eo-CSF) activity. Using anti murine (m) interleukin-5 (IL-5) antibody, we demonstrated that this activity was mainly derived from IL-5. Administration of prednisolone to patients decreased both Eo-CSF activity in sera and the number of eosinophils in blood. These results extend our recent study demonstrating that T cells from eosinophilic patients produce IL-5 with IL-2 stimulation and may support the speculation that IL-5 is an important factor which induces eosinophilia.