We have used differential display RT-PCR method to detect the genes specifically activated or repressed between mammary tumor and normal mammary epithelial cells. One of the genes identified is vimentin. The vimentin gene is abundantly expressed in both human and mouse mammary tumor cells and its expression decreased dramatically in normal mammary epithelial cells. The expression of vimentin gene correlates with the expression of transcription factor PEA3. Since the promoters of human and mouse vimentin genes contain one PEA3 binding site we investigated the ability of PEA3 to transactivate the vimentin promoter in mouse mammary epithelial cell CLS1, mouse mammary tumor MMT and human mammary tumor cell lines MCF7 and MDA231. Our results suggest that PEA3 specifically transactivates vimentin promoter through PEA3 site. Among members of the ETS transcription factor family only Erg showed ability to transactivate vimentin promoter besides PEA3. Our results also suggest that NFkB site on the vimentin promoter may act as a positive regulatory element for the transcription of vimentin. In metastatic mammary tumors derived from mice carrying the polyoma middle T or neu transgene, PEA3 is overexpressed and vimentin has been shown to play a key role in the motility of cells. Our results suggest that one of the roles of PEA3 in mammary tumor is to participate the activation of vimentin gene whose gene product in turn contributes to the metastatic potential of mammary tumors.