In the work described in this article, mouse fetal fibroblasts were treated with three lactation hormones (insulin, progesterone and oxytocin) and the cellular changes were analyzed by RT—PCR—Southern hybridization. A gene-expression pattern characteristic of mammary epithelioid cells was induced by the hormones, as indicated by expression of the marker genes α-casein and β-casein. Two mammary epithelial cell-specific gene expression vectors were constructed with bovine α-s1-casein or ovine β-casein gene promoters directing an EGFP reporter gene. Transient expression of the EGFP gene was observed in cells treated by the hormones but not in control cells. Cell morphology also changed after insulin and oxytocin treatments; the cells resembled epithelial cells rather than fibroblasts. Our results suggest that mouse fetus fibroblasts can be partially induced by lactation hormones to resemble mammary epithelial cells. This procedure might help to increase the efficiency of gene targeting in studies of mammary gland bioreactors.