Eosinophilia is a uniquely specific phenomenon regulated by interleukin-5 (IL-5), suggesting specific control for IL-5 expression. However in eosinophilia IL-5 is often co-expressed with other lymphokines such as IL-4, indicating that common, as well as independent, control mechanisms must exist. IL-5 gene expression is regulated at the transcriptional level. The molecular analysis of the IL-5 promoter region reveals the presence of positive regulatory sites that are common to many lymphokine genes. Results from immunosuppression studies suggest that the key control mechanism of IL-5 regulation may not depend on specific regulatory factors but on how gene expression is activated.