BACKGROUND: Interleukin-5 (IL-5) is responsible for eosinophilia in allergic diseases. In allergic bronchial asthma, there is a correlation between the extent of eosinophil infiltration in bronchial mucosa and IL-5 concentrations. In addition, IL-2 concentration is elevated in the airways and associated with eosinophilia in symptomatic patients with bronchial asthma. In animal studies, IL-2 can induce eosinophilia by increasing the synthesis of IL-5, however, it is still unknown how IL-2 can induce eosinophila in human being. The aim of this study is to evaluation the effect and mechanism of IL-2 on prolongation of eosinophil survival. METHODS: After purifiing the eosinophils from the venous blood of allergic patients with eosinophilia, we measured the survival rates of eosinophils using trypan blue dye exclusion test, and the number of eosinophils with Randolp's solution. We compared the survival rates of eosinophils in the presence of IL-2 or IL-5. Neutralizing antibody for IL-5 was added in IL-2 treated eosinophils to reveal whether IL-2 induced prolongation of eosinophil survival was mediated by IL-5. We checked IL-5 m-RNA expression of lymphocytes in the presence of IL-2 by using Reverse transcription-Polymerase chain reaction (RT-PCR) method to revealed the effect of IL-2 on IL-5 m-RNA expression on lymphocyte. alpha and beta IL-2 receptors were measured on eosinophils and lymphocytes with flow-cytometer after stimulated with IL-2. RESULTS: 1) Eosinophil survival rates increased dose dependently on IL-5 and IL-2. 2) The eosinophil survival rates increased by IL-2 were not inhibited by the pretreatment with neutralizing antibody for IL-5. 3) IL-5 m-RNA was not expressed on lymphocytes by the treatment with IL-2 up to 96 hours. 4) IL-2 upregulate the expression of IL-2Ralpha on eosinophils, instead of no effect on the expression of IL-2Rbeta. CONCLUSION: Interleukin-2 had the enhancing effect on the survival rates of eosinophils. The mechanism behind IL-2 induced eosinophilia might be the increment of IL-2 receptors on eosinophils rather than IL-5 synthesis by lymphocytes.