Granulocyte-macrophage-colony-stimulating factor (GM-CSF) and IL-3 are cytokines that have roles promoting the proliferation, differentiation, and function of numerous classes of hemopoietic cells and whose genes are closely linked in mice. To investigate the function of cyclosporine A-inhibitable enhancer located 2 kb upstream of the mouse GM-CSF gene on the transcriptional regulation of IL-3 gene, we isolated the enhancer locus from genomic DNA of mouse EL4 T cell line by PCR and inserted it into the previously cloned IL-3-CAT reporter plasmid (pCIL3-1003), and named as pEIL3-1003 (having enhancer). In transiently transfected mouse IC-2 mast cells, the promoter pCIL3-1003 was induced 15-fold or 2-fold in the presence of A23187 alone or A23187 plus PMA, however, in case of pEIL-1003, these inductions were enhanced as 40-fold or 55-fold respectively. This induction was in each case reduced to near basal levels in the presence of cyclosporin A. These results clearly show that this cyclosporin A sensitive enhancer region acts as an enhancer of the mouse IL-3 promoter.