Interleukin-5 (IL-5) plays a central role in the growth and differentiation of eosinophils and contributes to several disease states including asthma. There has been considerable interest in the identification of the transcriptional mechanisms controlling the synthesis of this cytokine. The regulation of IL-5 message is primarily at the level of transcription and is likely to be controlled, to a large extent, by regulatory elements in the promoter region that can influence the transcriptional activity of the gene. In this study, we performed a series of transient transfection experiments with IL-5 promoter-reporter gene construct and expression plasmid for E1A, an inhibitor of CBP/p300, or for the CBP/p300-binding defective E1A Delta 2-36. The results showed that E1A repressed IL-5 promoter activity, while E1A Delta 2-36 had no effect on it. This suggested that CBP/p300 was involved in regulation of IL-5 gene expression. Transcriptional coactivator CBP/p300 and transcription factors C/EBP, NF-AT, and c-Fos synergistically activated IL-5 promoter. Furthermore, we found that ectopic expression of p300 increased endogenous IL-5 mRNA expression. Thus, the histone acetyltransferase (HAT) activity of CBP/p300 was required to activate IL-5 expression. This report provided evidence, for the first time, that CBP/p300 was involved in IL-5 gene expression and the HAT activity was important in regulation of IL-5 expression.